feat(utils): Allow for explicit use of database replicas

[emyller]

Contributes to Flagsmith/flagsmith#5814.

This is meant to replace the PrimaryReplicaRouter functionality in the core API.

Changes

Introduces a utility using_database_replica that wraps any Django model manager binding it to a database replica according to settings.REPLICA_READ_STRATEGY.

Tests

Unit tests.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.51%. Comparing base (269aea1) to head (e880860).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #95      +/-   ##
==========================================
+ Coverage   94.08%   94.51%   +0.43%     
==========================================
  Files          73       75       +2     
  Lines        2232     2408     +176     
==========================================
+ Hits         2100     2276     +176     
  Misses        132      132              

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[khvn26]

Overall, looks great. Added a couple questions of moderate to low impact. Would like to see this tested in threaded Gunicorn to understand whether we need to use thread-local storage for the sequential replica cache. Besides that, interested to see if there's a performance effect on high-volume queries.

[khvn26]

Do we want to export Prometheus metrics for this?

e.g. flagsmith_db_chosen_replica_count labeled by status and replica_connection_name.

[emyller]

Do we want to export Prometheus metrics for this?

I'd be happy to do it given an example, and if it doesn't extend much of the scope here.

[khvn26]

LGTM 👍 Looking forward to seeing this used in Core.