chore(metrics): Rename _metrics to metrics

[alexander-alderman-webb]

Description

The team is preparing an announcement for the open beta of metrics.

Moves metrics code from _metrics.py to metrics.py, and updates metrics tests.

Issues

Reminders

• Please add tests to validate your changes, and lint your code using tox -e linters.
• Add GH Issue ID & Linear ID (if applicable)
• PR title should use conventional commit style (feat:, fix:, ref:, meta:)
• For external contributors: CONTRIBUTING.md, Sentry SDK development docs, Discord community

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 84.00%. Comparing base (104db8c) to head (e344c17).
⚠️ Report is 6 commits behind head on master.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #5035      +/-   ##
==========================================
+ Coverage   83.94%   84.00%   +0.06%     
==========================================
  Files         178      178              
  Lines       17834    17839       +5     
  Branches     3170     3171       +1     
==========================================
+ Hits        14971    14986      +15     
+ Misses       1901     1894       -7     
+ Partials      962      959       -3     

Files with missing lines	Coverage Δ
sentry_sdk/__init__.py	100.00% <100.00%> (ø)
sentry_sdk/metrics.py	100.00% <ø> (ø)

... and 6 files with indirect coverage changes