fix: Add hard limit to metrics batcher

[alexander-alderman-webb]

Description

Introduce a hard limit on the number of elements in the metrics batcher. The size constraint prevents the metrics batcher from using too much memory.

Issues

Closes #5055

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

❌ Patch coverage is 77.77778% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 83.97%. Comparing base (6a76cc5) to head (b6be967).
⚠️ Report is 3 commits behind head on master.
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
sentry_sdk/client.py	50.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #5068      +/-   ##
==========================================
+ Coverage   83.93%   83.97%   +0.04%     
==========================================
  Files         179      179              
  Lines       17928    17943      +15     
  Branches     3190     3192       +2     
==========================================
+ Hits        15047    15067      +20     
+ Misses       1909     1905       -4     
+ Partials      972      971       -1     

Files with missing lines	Coverage Δ
sentry_sdk/_metrics_batcher.py	73.86% <100.00%> (+3.98%)	⬆️
sentry_sdk/client.py	83.80% <50.00%> (-0.30%)	⬇️

... and 6 files with indirect coverage changes