[API] Announce stable logging API

[ThomsonTan]

New user-facing API is added in #2094, but all the logs features still marked as experimental which includes the new user-facing API. Could we evaluate our API with the aim of transitioning it to a stable stage? Once that's achieved, we could encourage users to add log instrumentation to broader scope.

[marcalff]

The following issue is still unresolved, and part of the logs api/sdk milestone:

• [Logs API/SDK] Drop include_trace_context parameter from LoggerProvider::GetLogger() #2104

Should it be fixed before the log api is declared stable ?

[lalitb]

Also refer - https://github.com/open-telemetry/opentelemetry-cpp/milestone/18

[ThomsonTan]

I'll go through spec matrix compliance on logging.