[API] Add Enabled method to Tracer

[psx95]

Fixes #3149

Changes

Adds a method to Tracer (ABI version >= 2) that allows the user to check if the tracer is enabled or not. A disabled tracer does not produce spans.

This change enhances tracer spec-compatibility.

For significant contributions please make sure you have completed the following items:

• CHANGELOG.md updated for non-trivial changes
• Unit tests have been added
• Changes in public API reviewed

[netlify]

✅ Deploy Preview for opentelemetry-cpp-api-docs canceled.

Name	Link
🔨 Latest commit	48a2fe3
🔍 Latest deploy log	https://app.netlify.com/sites/opentelemetry-cpp-api-docs/deploys/680712ca5bf31a0008510510

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 89.57%. Comparing base (674cc9e) to head (48a2fe3).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3357      +/-   ##
==========================================
+ Coverage   89.56%   89.57%   +0.01%     
==========================================
  Files         210      210              
  Lines        6502     6505       +3     
==========================================
+ Hits         5823     5826       +3     
  Misses        679      679              

Files with missing lines	Coverage Δ
api/include/opentelemetry/trace/noop.h	93.94% <100.00%> (+0.61%)	⬆️
api/include/opentelemetry/trace/tracer.h	100.00% <ø> (ø)
sdk/src/trace/tracer.cc	83.61% <100.00%> (ø)

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[marcalff]

LGTM, thanks for the fix.

[dbarker]

LGTM. Thanks