Add support for OTLP over HTTP/protobuf (honeycombio#279)

Adds support for ingesting OTLP requests over HTTP with protobuf data. Changs in this PR:

- Move the existing gRPC export handler and helpers to to otlp_trace.go and tests into otlp_trace_test.go
- Add HTTP handler postOTLP that retrieves honeycomb headers, decodes the request body into a OTLP
- ExportTraceRequest and hands to internal function to process request
- Update OTLP grpc Export handler to retrieve honeycomb headers from metadata and hand request to new internal function to process
- Adds tests to verify rejection behaviour for non-protobuf based requests to postOTLP and test to pass request to postOTLP handler endpoint

route/errors.go

@@ -33,6 +33,7 @@ var (
 	ErrUpstreamUnavailable = handlerError{nil, "upstream target unavailable", http.StatusServiceUnavailable, true, true}
 	ErrReqToEvent          = handlerError{nil, "failed to parse event", http.StatusBadRequest, false, true}
 	ErrBatchToEvent        = handlerError{nil, "failed to parse event within batch", http.StatusBadRequest, false, true}
+	ErrInvalidContentType  = handlerError{nil, "invalid content-type - only 'application/protobuf' is supported", http.StatusNotImplemented, false, true}
 )
 
 func (r *Router) handlerReturnWithError(w http.ResponseWriter, he handlerError, err error) {