[API] Introduce a new global level api.diag for internal Diagnostic Logging (Part 2)

[MSNev]

Introduction of new api.diag global logger as discussed during SIG meetings and linked to #1754 and Part 1 #1877

Basic outline

• Provide a top-level diagnostic logger from the API as api.diag with methods like api.diag.warn, api.diag.debug, etc.
• diag logger is no-op by default
• register a default logging implementation by calling api.diag.setLogger(loggingImpl)

Part 2

Will include breaking changes

• Once contrib is changed to use api.diag, these exports will be removed, this will require that they stop using local Logger instances
• This will also have breaking changes for
  • Configuration definitions that use api.Logger and core.LogLevel
  • Functions that currently get or pass api.Logger instances
  • The type of the environment OTEL_LOG_LEVEL

[MSNev]

Please assign to me as I can't assign it myself

This Part 2, will also wait until Part 1 #1877 is complete, released and the contrib repo updated

[dyladan]

@MSNev @obecny and I had a quick call about the diag logger. documenting decisions here for future reference:

1. Should we keep the log levels or reduce to just console levels?

Error, warn, info, debug, verbose, none/all

2. Do we want to pass loggers as part of the config of each component?

No. For now, components will call the global logger. This removes the OptionalDiagLogger and the getDiagLoggerFromConfig helpers. The logging global error handler will go to api.diag.error

3. Do we want to create a base class with setLogger, logger getter helpers? Do we want to have a component namespace for logging a single component?

We are going to leave this for a future enhancement. Existing getters for the logger will be changed to point to the api.diag logger. Other classes which don’t have existing getters will be changed to use api.diag directly at the point of use.

4. Keep the existing numbering from the existing PR to keep space between log levels to allow additional levels to be added in the future. This will break numeric compatibility with the old logger.

[MSNev]

This is included in v0.17