Implement HTTP(OTLP/HTTP+JSON Protocol) exporters

[owent]

Implement OTLP/HTTP+JSON exporter trace exporter and metrics exporters.

Refer to:

1. https://github.com/open-telemetry/opentelemetry-specification/blob/main/specification/protocol/otlp.md
2. https://github.com/open-telemetry/oteps/blob/main/text/0122-otlp-http-json.md
3. https://github.com/open-telemetry/opentelemetry-js/tree/main/packages/opentelemetry-web
4. https://github.com/open-telemetry/opentelemetry-collector/tree/main/receiver/otlpreceiver

I want to implement a HTTP exporter to fit OTLP/HTTP specification and do not depend on gRPC. And I found I need implement a Recordable which is almost the same as opentelemetry::exporter::otlp::OtlpRecordable . I'm wondering if we can create a extension (e.g. sdk_proto) and move opentelemetry::exporter::otlp::OtlpRecordable and all conversion codes from sdk to opentelemetry-proto into the this this extension? So that we can reuse the codes in more exporters.

@ThomsonTan

[lalitb]

Thanks, @owent, this would be really helpful.
Looking at the OTLP implementation for otel-go, which is similar to what you are proposing though it's finally published as a single package/release containing both grpc and http.

[owent]

If I commit codes inner otlp exporter, maybe we should split WITH_OTLP into WITH_OTLP_GRPC and WITH_OTLP_HTTP ? Some environments may just want http exporter and do'nt want to depend grpc.

[ThomsonTan]

Yes, I think split WITH_OTLP into 2 variables makes sense here. Thanks for contributing this.

[owent]

Closed by #810