Ensure that OTLP/JSON exporter uses correct capitalization of field names

[tigrannajaryan]

We have recently found that the OTLP spec was confusingly written and it was not clear what capitalization to use for trace id, span id fields.

The spec was clarified in this PR: https://github.com/open-telemetry/opentelemetry-proto/pull/468/files (it was considered a bug in the spec and we were allowed to fix it).

All fields (including traceId and spanId fields) in OTLP/JSON must use camelCase names in exporters.

I have not verified how the OTLP/JSON exporter behaves in this repository. If the exporter is not already using camelCase name please fix it, if it already is compliant with the spec please close this issue.

Note that Otel Collector currently accepts both camelCase and snake_case field names and will continue to do so for a while to give time for existing users to transition all existing non-compliant data sources to new versions that are OTLP spec-compliant.

[pmcollins]

I can take this.

edit: I'm not seeing an OTLP/JSON exporter, just OTLP Proto over gRPC or HTTP

[pmcollins]

Looks like OTLP JSON over HTTP is an open issue: #1003

[lzchen]

Since OTLP over JSON has not been implemeted yet, we will be closing this issue and it should theoretically be properly implemented once a pr is out for it.