fix: Fix data race issue on metrics Counters and Histograms

[jimmyjames]

Description

Fix data race issue in metrics.go caused by unlocked access to the Counters and Histograms maps.

References

Similar to fix provided in #136

Review Checklist

• I have clicked on "allow edits by maintainers".
• I have added documentation for new/changed functionality in this PR or in a PR to openfga.dev [Provide a link to any relevant PRs in the references section above]
• The correct base branch is being used, if not main
• I have added tests to validate that the change in functionality is working as expected

If you haven't done so yet, we would appreciate it if you could star the OpenFGA repository. :)

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 26.01%. Comparing base (9c2614b) to head (fed49e5).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #139      +/-   ##
==========================================
+ Coverage   25.98%   26.01%   +0.03%     
==========================================
  Files         102      102              
  Lines       12405    12411       +6     
==========================================
+ Hits         3223     3229       +6     
  Misses       8911     8911              
  Partials      271      271              

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