fix: metrics endpoint identity overrides

[Zaimwa9]

Thanks for submitting a PR! Please check the boxes below:

closes #6133

• I have added information to docs/ if required so people know about the feature!
• I have filled in the "Changes" section below?
• I have filled in the "How did you test this code" section below?
• I have used a Conventional Commit title for this Pull Request

Changes

• Changed from scanning all identities (EdgeIdentity.dynamowrapper) to using a partition key query (DynamoEnvironmentV2Wrapper:environment_id): n (features) threads versus single partition query
• use of set to check override in feature

How did you test this code?

• Updated tests
• Compared before versus after metrics summaries

[vercel]

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3 Skipped Deployments

Project	Deployment	Preview	Comments	Updated (UTC)
docs	Ignored	Preview		Oct 16, 2025 8:37am
flagsmith-frontend-preview	Ignored	Preview		Oct 16, 2025 8:37am
flagsmith-frontend-staging	Ignored	Preview		Oct 16, 2025 8:37am

[github-actions]

Docker builds report

Image	Build Status	Security report
ghcr.io/flagsmith/flagsmith-api-test:pr-6159	Finished ✅	Skipped
ghcr.io/flagsmith/flagsmith-e2e:pr-6159	Finished ✅	Skipped
ghcr.io/flagsmith/flagsmith-api:pr-6159	Finished ✅	Results ✅
ghcr.io/flagsmith/flagsmith:pr-6159	Finished ✅	Results ✅
ghcr.io/flagsmith/flagsmith-private-cloud:pr-6159	Finished ✅	Results ✅
ghcr.io/flagsmith/flagsmith-frontend:pr-6159	Finished ✅	Results ✅

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.00%. Comparing base (547139a) to head (565e2fe).
⚠️ Report is 3 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #6159   +/-   ##
=======================================
  Coverage   98.00%   98.00%           
=======================================
  Files        1278     1278           
  Lines       44983    44991    +8     
=======================================
+ Hits        44084    44092    +8     
  Misses        899      899           

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