Add configuration option for JSON data format in otlphttp client

[gmreads]

Hi,

As per discussions on slack with @Aneurysm9 , having JSON support for exporter in addition to protobuff is welcome.

I came up with this rough draft before actually working on code.
@Aneurysm9 Would appreciate your thoughts on this, is this the right direction?

Requirements

• Backwards compatible by setting defaults as protobuff
• Single config to set Content-Type header and send JSON to the receiver.

Proposed Interface

• Add a new option while configuring client

   client := otlptracehttp.NewClient(
   // default being protobuff, and the supported data formats somewhere in constants
       otlptracehttp.WithDataFormat("JSON")

Implementation Details

• Planning to make changes in uploadTraces function here

   // UploadTraces sends a batch of spans to the collector.
   func (d *client) UploadTraces(ctx context.Context, protoSpans []*tracepb.ResourceSpans) error {
    pbRequest := &coltracepb.ExportTraceServiceRequest{
    	ResourceSpans: protoSpans,
    }
        // Marshal to JSON based on d.cfg.dataFormat
    rawRequest, err := proto.Marshal(pbRequest) 
    if err != nil {
    	return err
    }

Unknowns

• Do proto files have proper struct tags for json ?
• What's the difference between proto3 and proto
• Why two different json tags are present ? ( json:"name=resource_spans" json:"name=resourceSpans")
  (in the ExportTraceServiceRequest struct

[pellared]

I guess for doing JSON serialization you should use https://pkg.go.dev/google.golang.org/protobuf/encoding/protojson

You can also look at https://github.com/open-telemetry/opentelemetry-collector/blob/main/pdata/ptrace/internal/ptracejson/json.go

I think it is also worth to mention the specification: https://github.com/open-telemetry/opentelemetry-specification/blob/main/specification/protocol/otlp.md#json-protobuf-encoding

[gmreads]

Yes I was planning to use the protojson package.

Thanks for the references, but they are primarily implementation concerns.

I wanted to take inputs on whether the proposed interface is acceptable to be merged upstream ?

[pellared]

In general looks fine. Maybe with WithEncoding instead of WithDataFormat. I think it would be good if it accepts something more or less like

type Encoding string

const (
   EncodingProtobuf Encoding = "protobuf"
   EncodingJson  = "json"