feat: logs api

[yuzurihaaa]

Which problem is this PR solving?

1. This is a continuation of feat: proto logs #202. Please Review that PR first before continuing on this PR due to huge change on proto generated code from the mentioned PR.

Fixes # (issue)

Short description of the change

1. Add OpenTelemetry Logs API layer.
2. I am trying to make it as close as possible to https://www.npmjs.com/package/@opentelemetry/api-logs
3. I export them in experimental_api.dart as in js, this feature is still in experimental, but experimental_api.dart is placed under lib/src/ instead of lib/ which will require defining import package:opentelemetry/lib/src/experimental_api.dart when need it. Let me know if I should place them under sdk.dart

How Has This Been Tested?

1. Unit test.

Checklist:

• Unit tests have been added
• Documentation has been updated

[yuzurihaaa]

branch rebased from latest master

[yuzurihaaa]

Hi @blakeroberts-wk , about unit test, seems like the part that is causing it to fail is

warning • lib/src/api/context/context.dart:22:18 • The declaration '_contextStackKey' isn't referenced. Try removing the declaration of '_contextStackKey'. •
          unused_element

I ran make analyze on local and I got the same error make: *** [analyze] Error 2

removing final ContextKey _contextStackKey = ContextKey(); , run make analyze and the error no more. I create another PR to fix CI when running make analyze.

[yuzurihaaa]

rebased with latest master

[blakeroberts-wk]

QA +1 experimental public or private API additions only, CI is sufficient

[blakeroberts-wk]

@Workiva/release-management-p

[rmconsole-wf]

+1 from RM