Allow users to pass an io.Writer to protojson

[denysvitali]

What language does this apply to?

Golang (protocolbuffers/protobuf-go)

Describe the problem you are trying to solve.
As part of open-telemetry/opentelemetry-collector#7095 we would like to switch to the new Protobuf Opaque API to increase the performance of opentelemetry-collector (specifically when exporting telemetry as JSON).

We would like to use encoding/protojson for this, but the current implementation doesn't allow users to pass an io.Writer.

The only method available to encode JSON is:

func Marshal(m proto.Message) ([]byte, error)

Describe the solution you'd like

We would like to see a solution similar to encoding/json's Encoder, where a new encoder can be created by passing an io.Writer and we can call Encode(obj).
This will prevent the use of a buffer that will only impact the encoding performance.

func NewEncoder(w io.Writer) *Encoder
func (enc *Encoder) Encode(v any) error

Describe alternatives you've considered

None

Additional context

• [pdata] Find replacement for now deprecated gogo/protobuf open-telemetry/opentelemetry-collector#7095
• https://pkg.go.dev/google.golang.org/[email protected]/encoding/protojson
• https://pkg.go.dev/encoding/json#Encoder

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cc/ @stapelberg, @els0r

[stapelberg]

I think this issue should have ended up in https://github.com/golang/protobuf/issues (the Go Protobuf issue tracker), but I lack permission to move it there.

@denysvitali Would you mind re-filing in the correct issue tracker please?

[denysvitali]

Sure thing, will do :)