[API] Add functionality in Tracer to report if it is enabled

[psx95]

OpenTelemetry Specification specifies that a Tracer SHOULD provide functions to report if it is enabled or not.

This spec change was added in PR#4063 and it goes well with the spec compliance changes requested in issue #2641.

[marcalff]

This is an API change in the specs, adding breaking change label.
To implement in ABI_VERSION 2.

[github-actions]

This issue was marked as stale due to lack of activity.