Be able to send OTLP/HTTP+JSON

[nullren]

The OTLP spec has three different ways to send OTLP data: OTLP over gRPC, OTLP over HTTP in binary, and OTLP over HTTP in JSON. It would be useful for otelgen to use any of the three to help exercise endpoints that support any of the three.

[nullren]

This might be a shortcoming of https://pkg.go.dev/go.opentelemetry.io/otel/exporters/otlp/otlpmetric since otlpmetrichttp seems to be only via protobuf payloads. That's what's currently used.

[krzko]

Hi @nullren , I'll scope it out. At present it it sends over gRPC and HTTP (JSON). Will see what it will take to sendind in HTTP binary format as well.