diff --git a/internal/gengapic/doc_file.go b/internal/gengapic/doc_file.go
index 527142c80..ccad64896 100644
--- a/internal/gengapic/doc_file.go
+++ b/internal/gengapic/doc_file.go
@@ -144,18 +144,6 @@ func (g *generator) genDocFile(year int, scopes []string, serv *descriptor.Servi
 	p("}")
 	p("")
 
-	p("func insertMetadata(ctx context.Context, mds ...metadata.MD) context.Context {")
-	p("  out, _ := metadata.FromOutgoingContext(ctx)")
-	p("  out = out.Copy()")
-	p("  for _, md := range mds {")
-	p("    for k, v := range md {")
-	p("      out[k] = append(out[k], v...)")
-	p("    }")
-	p("  }")
-	p("  return metadata.NewOutgoingContext(ctx, out)")
-	p("}")
-	p("")
-
 	p("// DefaultAuthScopes reports the default set of authentication scopes to use with this package.")
 	p("func DefaultAuthScopes() []string {")
 	p("  return []string{")
diff --git a/internal/gengapic/gengapic.go b/internal/gengapic/gengapic.go
index bb6be172d..0306bdfb2 100644
--- a/internal/gengapic/gengapic.go
+++ b/internal/gengapic/gengapic.go
@@ -324,7 +324,7 @@ func (g *generator) insertImplicitRequestHeaders(m *descriptor.MethodDescriptorP
 	// Trim the trailing comma and ampersand symbols.
 	f := formats.String()[:formats.Len()-1]
 	v := values.String()[:values.Len()-1]
-	g.printf("md := metadata.Pairs(\"x-goog-request-params\", fmt.Sprintf(%q,%s))", f, v)
+	g.printf("hds := []string{\"x-goog-request-params\", fmt.Sprintf(%q,%s)}", f, v)
 	return nil
 }
 
@@ -355,7 +355,7 @@ func (g *generator) insertDynamicRequestHeaders(m *descriptor.MethodDescriptorPr
 	g.printf("}")
 	g.printf(`routingHeaders = strings.TrimSuffix(routingHeaders, "&")`)
 	g.imports[pbinfo.ImportSpec{Path: "strings"}] = true
-	g.printf("md := metadata.Pairs(\"x-goog-request-params\", routingHeaders)")
+	g.printf(`hds := []string{"x-goog-request-params", routingHeaders}`)
 	g.imports[pbinfo.ImportSpec{Path: "regexp"}] = true
 	return nil
 }
@@ -380,8 +380,10 @@ func (g *generator) insertRequestHeaders(m *descriptor.MethodDescriptorProto, t
 		p("")
 		switch t {
 		case grpc:
-			p("ctx = insertMetadata(ctx, c.xGoogMetadata, md)")
+			p("hds = append(c.xGoogHeaders, hds...)")
+			p("ctx = gax.InsertMetadataIntoOutgoingContext(ctx, hds...)")
 		case rest:
+			p("md := metadata.Pairs(hds...)")
 			p(`headers := buildHeaders(ctx, c.xGoogMetadata, md, metadata.Pairs("Content-Type", "application/json"))`)
 		}
 		g.imports[pbinfo.ImportSpec{Path: "fmt"}] = true
@@ -391,7 +393,7 @@ func (g *generator) insertRequestHeaders(m *descriptor.MethodDescriptorProto, t
 	// No request-based header parameters.
 	switch t {
 	case grpc:
-		p("ctx = insertMetadata(ctx, c.xGoogMetadata)")
+		p("ctx = gax.InsertMetadataIntoOutgoingContext(ctx, c.xGoogHeaders...)")
 	case rest:
 		p(`headers := buildHeaders(ctx, c.xGoogMetadata, metadata.Pairs("Content-Type", "application/json"))`)
 	}
diff --git a/internal/gengapic/gengrpc.go b/internal/gengapic/gengrpc.go
index 6057a5135..bf291e695 100644
--- a/internal/gengapic/gengrpc.go
+++ b/internal/gengapic/gengrpc.go
@@ -288,7 +288,7 @@ func (g *generator) grpcClientInit(serv *descriptor.ServiceDescriptorProto, serv
 	g.mixinStubs()
 
 	p("// The x-goog-* metadata to be sent with each request.")
-	p("xGoogMetadata metadata.MD")
+	p("xGoogHeaders []string")
 
 	p("}")
 	p("")
@@ -378,7 +378,7 @@ func (g *generator) grpcClientUtilities(serv *descriptor.ServiceDescriptorProto,
 	p("func (c *%s) setGoogleClientInfo(keyval ...string) {", lowcaseServName)
 	p(`  kv := append([]string{"gl-go", gax.GoVersion}, keyval...)`)
 	p(`  kv = append(kv, "gapic", getVersionClient(), "gax", gax.Version, "grpc", grpc.Version)`)
-	p(`  c.xGoogMetadata = metadata.Pairs("x-goog-api-client", gax.XGoogHeader(kv...))`)
+	p(`  c.xGoogHeaders = []string{"x-goog-api-client", gax.XGoogHeader(kv...)}`)
 	p("}")
 	p("")
 
diff --git a/internal/gengapic/testdata/deprecated_client_init.want b/internal/gengapic/testdata/deprecated_client_init.want
index 5cd6fa2ff..0b4f29c77 100644
--- a/internal/gengapic/testdata/deprecated_client_init.want
+++ b/internal/gengapic/testdata/deprecated_client_init.want
@@ -63,7 +63,7 @@ type gRPCClient struct {
 	client mypackagepb.FooClient
 
 	// The x-goog-* metadata to be sent with each request.
-	xGoogMetadata metadata.MD
+	xGoogHeaders []string
 }
 
 // NewClient creates a new foo client based on gRPC.
@@ -115,7 +115,7 @@ func (c *gRPCClient) Connection() *grpc.ClientConn {
 func (c *gRPCClient) setGoogleClientInfo(keyval ...string) {
 	kv := append([]string{"gl-go", gax.GoVersion}, keyval...)
 	kv = append(kv, "gapic", getVersionClient(), "gax", gax.Version, "grpc", grpc.Version)
-	c.xGoogMetadata = metadata.Pairs("x-goog-api-client", gax.XGoogHeader(kv...))
+	c.xGoogHeaders = []string{"x-goog-api-client", gax.XGoogHeader(kv...)}
 }
 
 // Close closes the connection to the API service. The user should invoke this when
diff --git a/internal/gengapic/testdata/doc_file.want b/internal/gengapic/testdata/doc_file.want
index 41dbac2f1..91363764c 100644
--- a/internal/gengapic/testdata/doc_file.want
+++ b/internal/gengapic/testdata/doc_file.want
@@ -108,17 +108,6 @@ func getVersionClient() string {
 	return versionClient
 }
 
-func insertMetadata(ctx context.Context, mds ...metadata.MD) context.Context {
-	out, _ := metadata.FromOutgoingContext(ctx)
-	out = out.Copy()
-	for _, md := range mds {
-		for k, v := range md {
-			out[k] = append(out[k], v...)
-		}
-	}
-	return metadata.NewOutgoingContext(ctx, out)
-}
-
 // DefaultAuthScopes reports the default set of authentication scopes to use with this package.
 func DefaultAuthScopes() []string {
 	return []string{
diff --git a/internal/gengapic/testdata/doc_file_alpha.want b/internal/gengapic/testdata/doc_file_alpha.want
index 6dd3fad3e..f47b6ecf4 100644
--- a/internal/gengapic/testdata/doc_file_alpha.want
+++ b/internal/gengapic/testdata/doc_file_alpha.want
@@ -110,17 +110,6 @@ func getVersionClient() string {
 	return versionClient
 }
 
-func insertMetadata(ctx context.Context, mds ...metadata.MD) context.Context {
-	out, _ := metadata.FromOutgoingContext(ctx)
-	out = out.Copy()
-	for _, md := range mds {
-		for k, v := range md {
-			out[k] = append(out[k], v...)
-		}
-	}
-	return metadata.NewOutgoingContext(ctx, out)
-}
-
 // DefaultAuthScopes reports the default set of authentication scopes to use with this package.
 func DefaultAuthScopes() []string {
 	return []string{
diff --git a/internal/gengapic/testdata/doc_file_alpha_emptyservice.want b/internal/gengapic/testdata/doc_file_alpha_emptyservice.want
index c763ccc96..89ec7755e 100644
--- a/internal/gengapic/testdata/doc_file_alpha_emptyservice.want
+++ b/internal/gengapic/testdata/doc_file_alpha_emptyservice.want
@@ -82,17 +82,6 @@ func getVersionClient() string {
 	return versionClient
 }
 
-func insertMetadata(ctx context.Context, mds ...metadata.MD) context.Context {
-	out, _ := metadata.FromOutgoingContext(ctx)
-	out = out.Copy()
-	for _, md := range mds {
-		for k, v := range md {
-			out[k] = append(out[k], v...)
-		}
-	}
-	return metadata.NewOutgoingContext(ctx, out)
-}
-
 // DefaultAuthScopes reports the default set of authentication scopes to use with this package.
 func DefaultAuthScopes() []string {
 	return []string{
diff --git a/internal/gengapic/testdata/doc_file_beta.want b/internal/gengapic/testdata/doc_file_beta.want
index 71df94e0b..9842702ad 100644
--- a/internal/gengapic/testdata/doc_file_beta.want
+++ b/internal/gengapic/testdata/doc_file_beta.want
@@ -110,17 +110,6 @@ func getVersionClient() string {
 	return versionClient
 }
 
-func insertMetadata(ctx context.Context, mds ...metadata.MD) context.Context {
-	out, _ := metadata.FromOutgoingContext(ctx)
-	out = out.Copy()
-	for _, md := range mds {
-		for k, v := range md {
-			out[k] = append(out[k], v...)
-		}
-	}
-	return metadata.NewOutgoingContext(ctx, out)
-}
-
 // DefaultAuthScopes reports the default set of authentication scopes to use with this package.
 func DefaultAuthScopes() []string {
 	return []string{
diff --git a/internal/gengapic/testdata/doc_file_beta_emptyservice.want b/internal/gengapic/testdata/doc_file_beta_emptyservice.want
index 4a97784d1..294d47bf1 100644
--- a/internal/gengapic/testdata/doc_file_beta_emptyservice.want
+++ b/internal/gengapic/testdata/doc_file_beta_emptyservice.want
@@ -82,17 +82,6 @@ func getVersionClient() string {
 	return versionClient
 }
 
-func insertMetadata(ctx context.Context, mds ...metadata.MD) context.Context {
-	out, _ := metadata.FromOutgoingContext(ctx)
-	out = out.Copy()
-	for _, md := range mds {
-		for k, v := range md {
-			out[k] = append(out[k], v...)
-		}
-	}
-	return metadata.NewOutgoingContext(ctx, out)
-}
-
 // DefaultAuthScopes reports the default set of authentication scopes to use with this package.
 func DefaultAuthScopes() []string {
 	return []string{
diff --git a/internal/gengapic/testdata/doc_file_emptyservice.want b/internal/gengapic/testdata/doc_file_emptyservice.want
index 650b0be9e..1c262604d 100644
--- a/internal/gengapic/testdata/doc_file_emptyservice.want
+++ b/internal/gengapic/testdata/doc_file_emptyservice.want
@@ -80,17 +80,6 @@ func getVersionClient() string {
 	return versionClient
 }
 
-func insertMetadata(ctx context.Context, mds ...metadata.MD) context.Context {
-	out, _ := metadata.FromOutgoingContext(ctx)
-	out = out.Copy()
-	for _, md := range mds {
-		for k, v := range md {
-			out[k] = append(out[k], v...)
-		}
-	}
-	return metadata.NewOutgoingContext(ctx, out)
-}
-
 // DefaultAuthScopes reports the default set of authentication scopes to use with this package.
 func DefaultAuthScopes() []string {
 	return []string{
diff --git a/internal/gengapic/testdata/empty_client_init.want b/internal/gengapic/testdata/empty_client_init.want
index c106aa3dc..71551c430 100644
--- a/internal/gengapic/testdata/empty_client_init.want
+++ b/internal/gengapic/testdata/empty_client_init.want
@@ -61,7 +61,7 @@ type gRPCClient struct {
 	client mypackagepb.FooClient
 
 	// The x-goog-* metadata to be sent with each request.
-	xGoogMetadata metadata.MD
+	xGoogHeaders []string
 }
 
 // NewClient creates a new foo client based on gRPC.
@@ -111,7 +111,7 @@ func (c *gRPCClient) Connection() *grpc.ClientConn {
 func (c *gRPCClient) setGoogleClientInfo(keyval ...string) {
 	kv := append([]string{"gl-go", gax.GoVersion}, keyval...)
 	kv = append(kv, "gapic", getVersionClient(), "gax", gax.Version, "grpc", grpc.Version)
-	c.xGoogMetadata = metadata.Pairs("x-goog-api-client", gax.XGoogHeader(kv...))
+	c.xGoogHeaders = []string{"x-goog-api-client", gax.XGoogHeader(kv...)}
 }
 
 // Close closes the connection to the API service. The user should invoke this when
diff --git a/internal/gengapic/testdata/foo_client_init.want b/internal/gengapic/testdata/foo_client_init.want
index d0c091581..d552419e7 100644
--- a/internal/gengapic/testdata/foo_client_init.want
+++ b/internal/gengapic/testdata/foo_client_init.want
@@ -90,7 +90,7 @@ type fooGRPCClient struct {
 	locationsClient locationpb.LocationsClient
 
 	// The x-goog-* metadata to be sent with each request.
-	xGoogMetadata metadata.MD
+	xGoogHeaders []string
 }
 
 // NewFooClient creates a new foo client based on gRPC.
@@ -142,7 +142,7 @@ func (c *fooGRPCClient) Connection() *grpc.ClientConn {
 func (c *fooGRPCClient) setGoogleClientInfo(keyval ...string) {
 	kv := append([]string{"gl-go", gax.GoVersion}, keyval...)
 	kv = append(kv, "gapic", getVersionClient(), "gax", gax.Version, "grpc", grpc.Version)
-	c.xGoogMetadata = metadata.Pairs("x-goog-api-client", gax.XGoogHeader(kv...))
+	c.xGoogHeaders = []string{"x-goog-api-client", gax.XGoogHeader(kv...)}
 }
 
 // Close closes the connection to the API service. The user should invoke this when
diff --git a/internal/gengapic/testdata/lro_client_init.want b/internal/gengapic/testdata/lro_client_init.want
index e13769254..09a0d6245 100644
--- a/internal/gengapic/testdata/lro_client_init.want
+++ b/internal/gengapic/testdata/lro_client_init.want
@@ -105,7 +105,7 @@ type fooGRPCClient struct {
 	operationsClient longrunningpb.OperationsClient
 
 	// The x-goog-* metadata to be sent with each request.
-	xGoogMetadata metadata.MD
+	xGoogHeaders []string
 }
 
 // NewFooClient creates a new foo client based on gRPC.
@@ -167,7 +167,7 @@ func (c *fooGRPCClient) Connection() *grpc.ClientConn {
 func (c *fooGRPCClient) setGoogleClientInfo(keyval ...string) {
 	kv := append([]string{"gl-go", gax.GoVersion}, keyval...)
 	kv = append(kv, "gapic", getVersionClient(), "gax", gax.Version, "grpc", grpc.Version)
-	c.xGoogMetadata = metadata.Pairs("x-goog-api-client", gax.XGoogHeader(kv...))
+	c.xGoogHeaders = []string{"x-goog-api-client", gax.XGoogHeader(kv...)}
 }
 
 // Close closes the connection to the API service. The user should invoke this when
diff --git a/internal/gengapic/testdata/method_BidiThings.want b/internal/gengapic/testdata/method_BidiThings.want
index 3a4e9083d..7335ad111 100644
--- a/internal/gengapic/testdata/method_BidiThings.want
+++ b/internal/gengapic/testdata/method_BidiThings.want
@@ -1,5 +1,5 @@
 func (c *fooGRPCClient) BidiThings(ctx context.Context, opts ...gax.CallOption) (mypackagepb.Foo_BidiThingsClient, error) {
-	ctx = insertMetadata(ctx, c.xGoogMetadata)
+	ctx = gax.InsertMetadataIntoOutgoingContext(ctx, c.xGoogHeaders...)
 	var resp mypackagepb.Foo_BidiThingsClient
 	opts = append((*c.CallOptions).BidiThings[0:len((*c.CallOptions).BidiThings):len((*c.CallOptions).BidiThings)], opts...)
 	err := gax.Invoke(ctx, func(ctx context.Context, settings gax.CallSettings) error {
diff --git a/internal/gengapic/testdata/method_ClientThings.want b/internal/gengapic/testdata/method_ClientThings.want
index e9e3e9779..51a3d2a9d 100644
--- a/internal/gengapic/testdata/method_ClientThings.want
+++ b/internal/gengapic/testdata/method_ClientThings.want
@@ -1,5 +1,5 @@
 func (c *fooGRPCClient) ClientThings(ctx context.Context, opts ...gax.CallOption) (mypackagepb.Foo_ClientThingsClient, error) {
-	ctx = insertMetadata(ctx, c.xGoogMetadata)
+	ctx = gax.InsertMetadataIntoOutgoingContext(ctx, c.xGoogHeaders...)
 	var resp mypackagepb.Foo_ClientThingsClient
 	opts = append((*c.CallOptions).ClientThings[0:len((*c.CallOptions).ClientThings):len((*c.CallOptions).ClientThings)], opts...)
 	err := gax.Invoke(ctx, func(ctx context.Context, settings gax.CallSettings) error {
diff --git a/internal/gengapic/testdata/method_EmptyLRO.want b/internal/gengapic/testdata/method_EmptyLRO.want
index 964cc570a..a07842dda 100644
--- a/internal/gengapic/testdata/method_EmptyLRO.want
+++ b/internal/gengapic/testdata/method_EmptyLRO.want
@@ -1,5 +1,5 @@
 func (c *fooGRPCClient) EmptyLRO(ctx context.Context, req *mypackagepb.InputType, opts ...gax.CallOption) (*EmptyLROOperation, error) {
-	ctx = insertMetadata(ctx, c.xGoogMetadata)
+	ctx = gax.InsertMetadataIntoOutgoingContext(ctx, c.xGoogHeaders...)
 	opts = append((*c.CallOptions).EmptyLRO[0:len((*c.CallOptions).EmptyLRO):len((*c.CallOptions).EmptyLRO)], opts...)
 	var resp *longrunningpb.Operation
 	err := gax.Invoke(ctx, func(ctx context.Context, settings gax.CallSettings) error {
diff --git a/internal/gengapic/testdata/method_GetAnotherThing.want b/internal/gengapic/testdata/method_GetAnotherThing.want
index e03909c6e..c51f14b55 100644
--- a/internal/gengapic/testdata/method_GetAnotherThing.want
+++ b/internal/gengapic/testdata/method_GetAnotherThing.want
@@ -26,9 +26,10 @@ func (c *fooGRPCClient) GetAnotherThing(ctx context.Context, req *mypackagepb.In
 		routingHeaders = fmt.Sprintf("%s%s=%s&", routingHeaders, headerName, headerValue)
 	}
 	routingHeaders = strings.TrimSuffix(routingHeaders, "&")
-	md := metadata.Pairs("x-goog-request-params", routingHeaders)
+	hds := []string{"x-goog-request-params", routingHeaders}
 
-	ctx = insertMetadata(ctx, c.xGoogMetadata, md)
+	hds = append(c.xGoogHeaders, hds...)
+	ctx = gax.InsertMetadataIntoOutgoingContext(ctx, hds...)
 	opts = append((*c.CallOptions).GetAnotherThing[0:len((*c.CallOptions).GetAnotherThing):len((*c.CallOptions).GetAnotherThing)], opts...)
 	var resp *mypackagepb.OutputType
 	err := gax.Invoke(ctx, func(ctx context.Context, settings gax.CallSettings) error {
diff --git a/internal/gengapic/testdata/method_GetEmptyThing.want b/internal/gengapic/testdata/method_GetEmptyThing.want
index 631fbc53e..756cc49fe 100644
--- a/internal/gengapic/testdata/method_GetEmptyThing.want
+++ b/internal/gengapic/testdata/method_GetEmptyThing.want
@@ -1,7 +1,8 @@
 func (c *fooGRPCClient) GetEmptyThing(ctx context.Context, req *mypackagepb.InputType, opts ...gax.CallOption) error {
-	md := metadata.Pairs("x-goog-request-params", fmt.Sprintf("%s=%v&%s=%v&%s=%v&%s=%v&%s=%v&%s=%v", "field_name.nested", url.QueryEscape(req.GetFieldName().GetNested()), "other", url.QueryEscape(req.GetOther()), "another", url.QueryEscape(req.GetAnother()), "biz", url.QueryEscape(fmt.Sprintf("%g", req.GetBiz())), "top_level_enum", mypackagepb.TopLevelEnum_name[int32(req.GetTopLevelEnum())], "nested_enum", mypackagepb.InputType_NestedEnum_name[int32(req.GetNestedEnum())]))
+	hds := []string{"x-goog-request-params", fmt.Sprintf("%s=%v&%s=%v&%s=%v&%s=%v&%s=%v&%s=%v", "field_name.nested", url.QueryEscape(req.GetFieldName().GetNested()), "other", url.QueryEscape(req.GetOther()), "another", url.QueryEscape(req.GetAnother()), "biz", url.QueryEscape(fmt.Sprintf("%g", req.GetBiz())), "top_level_enum", mypackagepb.TopLevelEnum_name[int32(req.GetTopLevelEnum())], "nested_enum", mypackagepb.InputType_NestedEnum_name[int32(req.GetNestedEnum())])}
 
-	ctx = insertMetadata(ctx, c.xGoogMetadata, md)
+	hds = append(c.xGoogHeaders, hds...)
+	ctx = gax.InsertMetadataIntoOutgoingContext(ctx, hds...)
 	opts = append((*c.CallOptions).GetEmptyThing[0:len((*c.CallOptions).GetEmptyThing):len((*c.CallOptions).GetEmptyThing)], opts...)
 	err := gax.Invoke(ctx, func(ctx context.Context, settings gax.CallSettings) error {
 		var err error
diff --git a/internal/gengapic/testdata/method_GetManyOtherThings.want b/internal/gengapic/testdata/method_GetManyOtherThings.want
index 14cbe6a45..cb1bece89 100644
--- a/internal/gengapic/testdata/method_GetManyOtherThings.want
+++ b/internal/gengapic/testdata/method_GetManyOtherThings.want
@@ -1,5 +1,5 @@
 func (c *fooGRPCClient) GetManyOtherThings(ctx context.Context, req *mypackagepb.PageInputType, opts ...gax.CallOption) *StringIterator {
-	ctx = insertMetadata(ctx, c.xGoogMetadata)
+	ctx = gax.InsertMetadataIntoOutgoingContext(ctx, c.xGoogHeaders...)
 	opts = append((*c.CallOptions).GetManyOtherThings[0:len((*c.CallOptions).GetManyOtherThings):len((*c.CallOptions).GetManyOtherThings)], opts...)
 	it := &StringIterator{}
 	req = proto.Clone(req).(*mypackagepb.PageInputType)
diff --git a/internal/gengapic/testdata/method_GetManyThings.want b/internal/gengapic/testdata/method_GetManyThings.want
index 6c2785c96..94bcaca7c 100644
--- a/internal/gengapic/testdata/method_GetManyThings.want
+++ b/internal/gengapic/testdata/method_GetManyThings.want
@@ -1,7 +1,8 @@
 func (c *fooGRPCClient) GetManyThings(ctx context.Context, req *mypackagepb.PageInputType, opts ...gax.CallOption) *StringIterator {
-	md := metadata.Pairs("x-goog-request-params", fmt.Sprintf("%s=%v&%s=%v&%s=%v&%s=%v&%s=%v&%s=%v", "field_name.nested", url.QueryEscape(req.GetFieldName().GetNested()), "other", url.QueryEscape(req.GetOther()), "another", url.QueryEscape(req.GetAnother()), "biz", url.QueryEscape(fmt.Sprintf("%g", req.GetBiz())), "top_level_enum", mypackagepb.TopLevelEnum_name[int32(req.GetTopLevelEnum())], "nested_enum", mypackagepb.InputType_NestedEnum_name[int32(req.GetNestedEnum())]))
+	hds := []string{"x-goog-request-params", fmt.Sprintf("%s=%v&%s=%v&%s=%v&%s=%v&%s=%v&%s=%v", "field_name.nested", url.QueryEscape(req.GetFieldName().GetNested()), "other", url.QueryEscape(req.GetOther()), "another", url.QueryEscape(req.GetAnother()), "biz", url.QueryEscape(fmt.Sprintf("%g", req.GetBiz())), "top_level_enum", mypackagepb.TopLevelEnum_name[int32(req.GetTopLevelEnum())], "nested_enum", mypackagepb.InputType_NestedEnum_name[int32(req.GetNestedEnum())])}
 
-	ctx = insertMetadata(ctx, c.xGoogMetadata, md)
+	hds = append(c.xGoogHeaders, hds...)
+	ctx = gax.InsertMetadataIntoOutgoingContext(ctx, hds...)
 	opts = append((*c.CallOptions).GetManyThings[0:len((*c.CallOptions).GetManyThings):len((*c.CallOptions).GetManyThings)], opts...)
 	it := &StringIterator{}
 	req = proto.Clone(req).(*mypackagepb.PageInputType)
diff --git a/internal/gengapic/testdata/method_GetManyThingsOptional.want b/internal/gengapic/testdata/method_GetManyThingsOptional.want
index 4f633bc91..4e6a05289 100644
--- a/internal/gengapic/testdata/method_GetManyThingsOptional.want
+++ b/internal/gengapic/testdata/method_GetManyThingsOptional.want
@@ -1,7 +1,8 @@
 func (c *fooGRPCClient) GetManyThingsOptional(ctx context.Context, req *mypackagepb.PageInputTypeOptional, opts ...gax.CallOption) *StringIterator {
-	md := metadata.Pairs("x-goog-request-params", fmt.Sprintf("%s=%v&%s=%v&%s=%v&%s=%v&%s=%v&%s=%v", "field_name.nested", url.QueryEscape(req.GetFieldName().GetNested()), "other", url.QueryEscape(req.GetOther()), "another", url.QueryEscape(req.GetAnother()), "biz", url.QueryEscape(fmt.Sprintf("%g", req.GetBiz())), "top_level_enum", mypackagepb.TopLevelEnum_name[int32(req.GetTopLevelEnum())], "nested_enum", mypackagepb.InputType_NestedEnum_name[int32(req.GetNestedEnum())]))
+	hds := []string{"x-goog-request-params", fmt.Sprintf("%s=%v&%s=%v&%s=%v&%s=%v&%s=%v&%s=%v", "field_name.nested", url.QueryEscape(req.GetFieldName().GetNested()), "other", url.QueryEscape(req.GetOther()), "another", url.QueryEscape(req.GetAnother()), "biz", url.QueryEscape(fmt.Sprintf("%g", req.GetBiz())), "top_level_enum", mypackagepb.TopLevelEnum_name[int32(req.GetTopLevelEnum())], "nested_enum", mypackagepb.InputType_NestedEnum_name[int32(req.GetNestedEnum())])}
 
-	ctx = insertMetadata(ctx, c.xGoogMetadata, md)
+	hds = append(c.xGoogHeaders, hds...)
+	ctx = gax.InsertMetadataIntoOutgoingContext(ctx, hds...)
 	opts = append((*c.CallOptions).GetManyThingsOptional[0:len((*c.CallOptions).GetManyThingsOptional):len((*c.CallOptions).GetManyThingsOptional)], opts...)
 	it := &StringIterator{}
 	req = proto.Clone(req).(*mypackagepb.PageInputTypeOptional)
diff --git a/internal/gengapic/testdata/method_GetOneThing.want b/internal/gengapic/testdata/method_GetOneThing.want
index 6921e33cb..ac0ba44fd 100644
--- a/internal/gengapic/testdata/method_GetOneThing.want
+++ b/internal/gengapic/testdata/method_GetOneThing.want
@@ -1,7 +1,8 @@
 func (c *fooGRPCClient) GetOneThing(ctx context.Context, req *mypackagepb.InputType, opts ...gax.CallOption) (*mypackagepb.OutputType, error) {
-	md := metadata.Pairs("x-goog-request-params", fmt.Sprintf("%s=%v&%s=%v&%s=%v&%s=%v&%s=%v&%s=%v", "field_name.nested", url.QueryEscape(req.GetFieldName().GetNested()), "other", url.QueryEscape(req.GetOther()), "another", url.QueryEscape(req.GetAnother()), "biz", url.QueryEscape(fmt.Sprintf("%g", req.GetBiz())), "top_level_enum", mypackagepb.TopLevelEnum_name[int32(req.GetTopLevelEnum())], "nested_enum", mypackagepb.InputType_NestedEnum_name[int32(req.GetNestedEnum())]))
+	hds := []string{"x-goog-request-params", fmt.Sprintf("%s=%v&%s=%v&%s=%v&%s=%v&%s=%v&%s=%v", "field_name.nested", url.QueryEscape(req.GetFieldName().GetNested()), "other", url.QueryEscape(req.GetOther()), "another", url.QueryEscape(req.GetAnother()), "biz", url.QueryEscape(fmt.Sprintf("%g", req.GetBiz())), "top_level_enum", mypackagepb.TopLevelEnum_name[int32(req.GetTopLevelEnum())], "nested_enum", mypackagepb.InputType_NestedEnum_name[int32(req.GetNestedEnum())])}
 
-	ctx = insertMetadata(ctx, c.xGoogMetadata, md)
+	hds = append(c.xGoogHeaders, hds...)
+	ctx = gax.InsertMetadataIntoOutgoingContext(ctx, hds...)
 	opts = append((*c.CallOptions).GetOneThing[0:len((*c.CallOptions).GetOneThing):len((*c.CallOptions).GetOneThing)], opts...)
 	var resp *mypackagepb.OutputType
 	err := gax.Invoke(ctx, func(ctx context.Context, settings gax.CallSettings) error {
diff --git a/internal/gengapic/testdata/method_RespLRO.want b/internal/gengapic/testdata/method_RespLRO.want
index 35170d336..9c273369b 100644
--- a/internal/gengapic/testdata/method_RespLRO.want
+++ b/internal/gengapic/testdata/method_RespLRO.want
@@ -1,5 +1,5 @@
 func (c *fooGRPCClient) RespLRO(ctx context.Context, req *mypackagepb.InputType, opts ...gax.CallOption) (*RespLROOperation, error) {
-	ctx = insertMetadata(ctx, c.xGoogMetadata)
+	ctx = gax.InsertMetadataIntoOutgoingContext(ctx, c.xGoogHeaders...)
 	opts = append((*c.CallOptions).RespLRO[0:len((*c.CallOptions).RespLRO):len((*c.CallOptions).RespLRO)], opts...)
 	var resp *longrunningpb.Operation
 	err := gax.Invoke(ctx, func(ctx context.Context, settings gax.CallSettings) error {
diff --git a/internal/gengapic/testdata/method_ServerThings.want b/internal/gengapic/testdata/method_ServerThings.want
index b6172de96..75514616b 100644
--- a/internal/gengapic/testdata/method_ServerThings.want
+++ b/internal/gengapic/testdata/method_ServerThings.want
@@ -1,7 +1,8 @@
 func (c *fooGRPCClient) ServerThings(ctx context.Context, req *mypackagepb.InputType, opts ...gax.CallOption) (mypackagepb.Foo_ServerThingsClient, error) {
-	md := metadata.Pairs("x-goog-request-params", fmt.Sprintf("%s=%v&%s=%v&%s=%v&%s=%v&%s=%v&%s=%v", "field_name.nested", url.QueryEscape(req.GetFieldName().GetNested()), "other", url.QueryEscape(req.GetOther()), "another", url.QueryEscape(req.GetAnother()), "biz", url.QueryEscape(fmt.Sprintf("%g", req.GetBiz())), "top_level_enum", mypackagepb.TopLevelEnum_name[int32(req.GetTopLevelEnum())], "nested_enum", mypackagepb.InputType_NestedEnum_name[int32(req.GetNestedEnum())]))
+	hds := []string{"x-goog-request-params", fmt.Sprintf("%s=%v&%s=%v&%s=%v&%s=%v&%s=%v&%s=%v", "field_name.nested", url.QueryEscape(req.GetFieldName().GetNested()), "other", url.QueryEscape(req.GetOther()), "another", url.QueryEscape(req.GetAnother()), "biz", url.QueryEscape(fmt.Sprintf("%g", req.GetBiz())), "top_level_enum", mypackagepb.TopLevelEnum_name[int32(req.GetTopLevelEnum())], "nested_enum", mypackagepb.InputType_NestedEnum_name[int32(req.GetNestedEnum())])}
 
-	ctx = insertMetadata(ctx, c.xGoogMetadata, md)
+	hds = append(c.xGoogHeaders, hds...)
+	ctx = gax.InsertMetadataIntoOutgoingContext(ctx, hds...)
 	opts = append((*c.CallOptions).ServerThings[0:len((*c.CallOptions).ServerThings):len((*c.CallOptions).ServerThings)], opts...)
 	var resp mypackagepb.Foo_ServerThingsClient
 	err := gax.Invoke(ctx, func(ctx context.Context, settings gax.CallSettings) error {
diff --git a/internal/gengapic/testdata/rest_EmptyRPC.want b/internal/gengapic/testdata/rest_EmptyRPC.want
index 43d1a88f5..c1c9d2826 100644
--- a/internal/gengapic/testdata/rest_EmptyRPC.want
+++ b/internal/gengapic/testdata/rest_EmptyRPC.want
@@ -11,8 +11,9 @@ func (c *fooRESTClient) EmptyRPC(ctx context.Context, req *foopb.Foo, opts ...ga
 	baseUrl.RawQuery = params.Encode()
 
 	// Build HTTP headers from client and context metadata.
-	md := metadata.Pairs("x-goog-request-params", fmt.Sprintf("%s=%v", "other", url.QueryEscape(req.GetOther())))
+	hds := []string{"x-goog-request-params", fmt.Sprintf("%s=%v", "other", url.QueryEscape(req.GetOther()))}
 
+	md := metadata.Pairs(hds...)
 	headers := buildHeaders(ctx, c.xGoogMetadata, md, metadata.Pairs("Content-Type", "application/json"))
 	return gax.Invoke(ctx, func(ctx context.Context, settings gax.CallSettings) error {
 		if settings.Path != "" {
diff --git a/internal/gengapic/testdata/rest_HttpBodyRPC.want b/internal/gengapic/testdata/rest_HttpBodyRPC.want
index 636e56eb1..81c58561d 100644
--- a/internal/gengapic/testdata/rest_HttpBodyRPC.want
+++ b/internal/gengapic/testdata/rest_HttpBodyRPC.want
@@ -21,8 +21,9 @@ func (c *fooRESTClient) HttpBodyRPC(ctx context.Context, req *foopb.Foo, opts ..
 		routingHeaders = fmt.Sprintf("%s%s=%s&", routingHeaders, headerName, headerValue)
 	}
 	routingHeaders = strings.TrimSuffix(routingHeaders, "&")
-	md := metadata.Pairs("x-goog-request-params", routingHeaders)
+	hds := []string{"x-goog-request-params", routingHeaders}
 
+	md := metadata.Pairs(hds...)
 	headers := buildHeaders(ctx, c.xGoogMetadata, md, metadata.Pairs("Content-Type", "application/json"))
 	opts = append((*c.CallOptions).HttpBodyRPC[0:len((*c.CallOptions).HttpBodyRPC):len((*c.CallOptions).HttpBodyRPC)], opts...)
 	resp := &httpbodypb.HttpBody{}
diff --git a/internal/gengapic/testdata/rest_ServerStreamRPC.want b/internal/gengapic/testdata/rest_ServerStreamRPC.want
index 5ea373f0f..b29079ceb 100644
--- a/internal/gengapic/testdata/rest_ServerStreamRPC.want
+++ b/internal/gengapic/testdata/rest_ServerStreamRPC.want
@@ -21,8 +21,9 @@ func (c *fooRESTClient) ServerStreamRPC(ctx context.Context, req *foopb.Foo, opt
 		routingHeaders = fmt.Sprintf("%s%s=%s&", routingHeaders, headerName, headerValue)
 	}
 	routingHeaders = strings.TrimSuffix(routingHeaders, "&")
-	md := metadata.Pairs("x-goog-request-params", routingHeaders)
+	hds := []string{"x-goog-request-params", routingHeaders}
 
+	md := metadata.Pairs(hds...)
 	headers := buildHeaders(ctx, c.xGoogMetadata, md, metadata.Pairs("Content-Type", "application/json"))
 	var streamClient *serverStreamRPCRESTClient
 	e := gax.Invoke(ctx, func(ctx context.Context, settings gax.CallSettings) error {
diff --git a/internal/gengapic/testdata/rest_UnaryRPC.want b/internal/gengapic/testdata/rest_UnaryRPC.want
index 595246769..37a5bfb6e 100644
--- a/internal/gengapic/testdata/rest_UnaryRPC.want
+++ b/internal/gengapic/testdata/rest_UnaryRPC.want
@@ -26,8 +26,9 @@ func (c *fooRESTClient) UnaryRPC(ctx context.Context, req *foopb.Foo, opts ...ga
 		routingHeaders = fmt.Sprintf("%s%s=%s&", routingHeaders, headerName, headerValue)
 	}
 	routingHeaders = strings.TrimSuffix(routingHeaders, "&")
-	md := metadata.Pairs("x-goog-request-params", routingHeaders)
+	hds := []string{"x-goog-request-params", routingHeaders}
 
+	md := metadata.Pairs(hds...)
 	headers := buildHeaders(ctx, c.xGoogMetadata, md, metadata.Pairs("Content-Type", "application/json"))
 	opts = append((*c.CallOptions).UnaryRPC[0:len((*c.CallOptions).UnaryRPC):len((*c.CallOptions).UnaryRPC)], opts...)
 	unm := protojson.UnmarshalOptions{AllowPartial: true, DiscardUnknown: true}
diff --git a/showcase/go.mod b/showcase/go.mod
index 65ec0583c..87cb6fd73 100644
--- a/showcase/go.mod
+++ b/showcase/go.mod
@@ -32,3 +32,5 @@ require (
 	google.golang.org/appengine v1.6.7 // indirect
 	google.golang.org/genproto/googleapis/api v0.0.0-20230629202037-9506855d4529 // indirect
 )
+
+replace github.com/googleapis/gapic-showcase => ./gen/github.com/googleapis/gapic-showcase