Field registry: Don't overwrite fields with same name (elastic#13914) (elastic#14037)

When fields are registered (fields.go, autogenerated), they are grouped
by (Beatname, Name and Priority). Currently, if the same triplet is used
more than once, the previous asset is overwritten.

This is the case with Filebeat inputs and modules sharing the same name:

module/netflow/fields.go:
  asset.SetFields("filebeat", "netflow", asset.ModuleFieldsPri, AssetNetflow)

input/netflow/fields.go:
  asset.SetFields("filebeat", "netflow", asset.ModuleFieldsPri, AssetNetflow)

Also this introduces an inconsistence as the order of registration can
vary.

Fixes elastic#13768

(cherry picked from commit 5fec6bc)

Author: adriansr

CHANGELOG.next.asciidoc

@@ -51,6 +51,7 @@ https://github.com/elastic/beats/compare/v7.0.0-alpha2...master[Check the HEAD d
 - cisco asa and ftd filesets: Fix parsing of message 106001. {issue}13891[13891] {pull}13903[13903]
 - Fix merging of fields specified in global scope with fields specified under an input's scope. {issue}3628[3628] {pull}13909[13909]
 - Fix delay in enforcing close_renamed and close_removed options. {issue}13488[13488] {pull}13907[13907]
+- Fix missing netflow fields in index template. {issue}13768[13768] {pull}13914[13914]
 
 *Heartbeat*
 

libbeat/asset/registry.go

@@ -29,7 +29,7 @@ import (
 // As each entry is an array of bytes multiple fields.yml can be added under one path.
 // This can become useful as we don't have to generate anymore the fields.yml but can
 // package each local fields.yml from things like processors.
-var FieldsRegistry = map[string]map[int]map[string]string{}
+var FieldsRegistry = map[string]map[int]map[string][]string{}
 
 // SetFields sets the fields for a given beat and asset name
 func SetFields(beat, name string, p Priority, asset func() string) error {
@@ -38,14 +38,14 @@ func SetFields(beat, name string, p Priority, asset func() string) error {
 	priority := int(p)
 
 	if _, ok := FieldsRegistry[beat]; !ok {
-		FieldsRegistry[beat] = map[int]map[string]string{}
+		FieldsRegistry[beat] = map[int]map[string][]string{}
 	}
 
 	if _, ok := FieldsRegistry[beat][priority]; !ok {
-		FieldsRegistry[beat][priority] = map[string]string{}
+		FieldsRegistry[beat][priority] = map[string][]string{}
 	}
 
-	FieldsRegistry[beat][priority][name] = data
+	FieldsRegistry[beat][priority][name] = append(FieldsRegistry[beat][priority][name], data)
 
 	return nil
 }
@@ -74,13 +74,15 @@ func GetFields(beat string) ([]byte, error) {
 		sort.Strings(entries)
 
 		for _, entry := range entries {
-			data := priorityRegistry[entry]
-			output, err := DecodeData(data)
-			if err != nil {
-				return nil, err
+			list := priorityRegistry[entry]
+			for _, data := range list {
+				output, err := DecodeData(data)
+				if err != nil {
+					return nil, err
+				}
+
+				fields = append(fields, output...)
 			}
-
-			fields = append(fields, output...)
 		}
 	}
 	return fields, nil