feat: non-experimental before_send_metric option

[alexander-alderman-webb]

Description

Add a stable before_send_metric option. If no value is specified, the callback falls back onto the existing experimental callback.

Avoid using experimental options before metrics gains more adoption.

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 83.95%. Comparing base (7264a9f) to head (ec7cd14).
⚠️ Report is 2 commits behind head on master.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #5064      +/-   ##
==========================================
+ Coverage   83.94%   83.95%   +0.01%     
==========================================
  Files         179      179              
  Lines       17904    17922      +18     
  Branches     3179     3188       +9     
==========================================
+ Hits        15030    15047      +17     
  Misses       1902     1902              
- Partials      972      973       +1     

Files with missing lines	Coverage Δ
sentry_sdk/consts.py	99.40% <ø> (+<0.01%)	⬆️
sentry_sdk/utils.py	86.42% <100.00%> (ø)

... and 2 files with indirect coverage changes