add support for `http/json` configuration option

[codeboten]

Problem Statement

The OTLP exporter currently supports grpc and http/protobuf. This issue is to add support for http/json as well to support all the options from the specification.

Proposed Solution

Similar to the other options, the solution would be to add one more supported config option along with the tests for it.

Alternatives

Don't support http/json

Prior Art

Support for http/protobuf and grpc can be used as examples for implementing this.

Additional Context

See OTLP exporter options here: https://github.com/open-telemetry/opentelemetry-specification/blob/main/specification/protocol/exporter.md#configuration-options

[mattsains]

Am I correct in thinking that the underlying otelhttpclient doesn't support json emission yet? Judging from here: https://github.com/open-telemetry/opentelemetry-go/blob/main/exporters/otlp/otlplog/otlploghttp/client.go#L127C2-L127C39 where there is an unconditional conversion to protobuf bytes.

I'm guessing we'd need to add that before being able to configure it to work with json