otlpmetrichttp otlptracehttp: Handle non-protobuf responses

[pellared]

From: https://github.com/open-telemetry/opentelemetry-proto/blob/main/docs/specification.md#binary-protobuf-encoding

The client and the server MUST set "Content-Type: application/x-protobuf" request and response headers when sending binary Protobuf encoded payload.

Currently the exporter expects that the server to always respond with binary Protobuf encoded payload, but it may return something else e.g. text/plain with OK content.

See:

• opentelemetry-go/exporters/otlp/otlptrace/otlptracehttp/client.go

  Lines 181 to 185 in d968509

  	if respData.Len() != 0 {
  	var respProto coltracepb.ExportTraceServiceResponse
  	if err := proto.Unmarshal(respData.Bytes(), &respProto); err != nil {
  	return err
  	}

• opentelemetry-go/exporters/otlp/otlpmetric/otlpmetrichttp/client.go

  Lines 170 to 172 in d968509

  	if respData.Len() != 0 {
  	var respProto colmetricpb.ExportMetricsServiceResponse
  	if err := proto.Unmarshal(respData.Bytes(), &respProto); err != nil {