ref(metrics): Deprecate sentry_sdk.metrics

[szokeasaurusrex]

Raise a DeprecationWarning on import of the sentry_sdk.metrics module.

Closes #3502

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 79.74%. Comparing base (22f62b0) to head (42e6105).
Report is 1 commits behind head on master.

✅ All tests successful. No failed tests found.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3512      +/-   ##
==========================================
+ Coverage   79.72%   79.74%   +0.01%     
==========================================
  Files         136      136              
  Lines       14677    14681       +4     
  Branches     3082     3083       +1     
==========================================
+ Hits        11701    11707       +6     
+ Misses       2135     2134       -1     
+ Partials      841      840       -1     

Files with missing lines	Coverage Δ
sentry_sdk/metrics.py	81.29% <100.00%> (+0.08%)	⬆️
sentry_sdk/tracing.py	76.95% <100.00%> (+0.09%)	⬆️

... and 3 files with indirect coverage changes