feat(experimentation): accept inline metrics on experiment creation

[Zaimwa9]

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• I have read the Contributing Guide.
• 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.

Changes

POST /environments/{env_key}/experiments/ now accepts an optional
metrics: [{"metric": <id>, "expected_direction": <choice>}] list, so an experiment
and its metrics are created in a single transaction.

• Validation mirrors the nested attach endpoint: metrics must belong to the experiment's
  environment, duplicates are rejected, and metrics is rejected on PATCH.
• Experiment list/retrieve responses now include the attached metrics (read-only),
  prefetched to avoid N+1 queries.
• The field is optional — existing API consumers are unaffected.

How did you test this code?

• New 7 tests.

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3 Skipped Deployments

Project	Deployment	Actions	Updated (UTC)
docs	Ignored	Preview	Jun 15, 2026 8:55am
flagsmith-frontend-preview	Ignored	Preview	Jun 15, 2026 8:55am
flagsmith-frontend-staging	Ignored	Preview	Jun 15, 2026 8:55am

[github-actions]

Docker builds report

Image	Build Status	Security report
ghcr.io/flagsmith/flagsmith-e2e:pr-7770	Finished ✅	Skipped
ghcr.io/flagsmith/flagsmith-api-test:pr-7770	Finished ✅	Skipped
ghcr.io/flagsmith/flagsmith-frontend:pr-7770	Finished ✅	Results ✅
ghcr.io/flagsmith/flagsmith-api:pr-7770	Finished ✅	Results ✅
ghcr.io/flagsmith/flagsmith:pr-7770	Finished ✅	Results ✅
ghcr.io/flagsmith/flagsmith-private-cloud:pr-7770	Finished ✅	Results ✅

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.57%. Comparing base (a65d175) to head (34201ae).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #7770   +/-   ##
=======================================
  Coverage   98.56%   98.57%           
=======================================
  Files        1460     1460           
  Lines       56361    56443   +82     
=======================================
+ Hits        55554    55636   +82     
  Misses        807      807           

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[github-actions]

Playwright Test Results (oss - depot-ubuntu-latest-16)

 1 passed

Details

 1 test across 1 suite
 33.1 seconds
 846e4b0
 🔄 Run: #17466 (attempt 1)

Playwright Test Results (oss - depot-ubuntu-latest-arm-16)

 1 passed

Details

 1 test across 1 suite
 45.8 seconds
 846e4b0
 🔄 Run: #17466 (attempt 1)

Playwright Test Results (private-cloud - depot-ubuntu-latest-16)

 3 passed

Details

 3 tests across 3 suites
 40.5 seconds
 846e4b0
 🔄 Run: #17466 (attempt 1)

Playwright Test Results (private-cloud - depot-ubuntu-latest-arm-16)

 3 passed

Details

 3 tests across 3 suites
 41.5 seconds
 846e4b0
 🔄 Run: #17466 (attempt 1)

Playwright Test Results (oss - depot-ubuntu-latest-arm-16)

 1 passed

Details

 1 test across 1 suite
 37.9 seconds
 d9e4669
 🔄 Run: #17468 (attempt 1)

Playwright Test Results (oss - depot-ubuntu-latest-16)

 1 passed

Details

 1 test across 1 suite
 40.8 seconds
 d9e4669
 🔄 Run: #17468 (attempt 1)

Playwright Test Results (private-cloud - depot-ubuntu-latest-16)

 2 passed

Details

 2 tests across 2 suites
 33.5 seconds
 d9e4669
 🔄 Run: #17468 (attempt 1)

Playwright Test Results (private-cloud - depot-ubuntu-latest-arm-16)

 1 passed

Details

 1 test across 1 suite
 43 seconds
 d9e4669
 🔄 Run: #17468 (attempt 1)

Playwright Test Results (oss - depot-ubuntu-latest-16)

 1 passed

Details

 1 test across 1 suite
 40.8 seconds
 d594844
 🔄 Run: #17483 (attempt 1)

Playwright Test Results (oss - depot-ubuntu-latest-arm-16)

 1 passed

Details

 1 test across 1 suite
 44.9 seconds
 d594844
 🔄 Run: #17483 (attempt 1)

Playwright Test Results (private-cloud - depot-ubuntu-latest-16)

 3 passed

Details

 3 tests across 3 suites
 32.9 seconds
 d594844
 🔄 Run: #17483 (attempt 1)

Playwright Test Results (private-cloud - depot-ubuntu-latest-arm-16)

 1 passed

Details

 1 test across 1 suite
 1 minute, 9 seconds
 d594844
 🔄 Run: #17483 (attempt 1)

Playwright Test Results (oss - depot-ubuntu-latest-16)

 1 passed

Details

 1 test across 1 suite
 39.7 seconds
 34201ae
 🔄 Run: #17486 (attempt 1)

Playwright Test Results (oss - depot-ubuntu-latest-arm-16)

 1 passed

Details

 1 test across 1 suite
 44 seconds
 34201ae
 🔄 Run: #17486 (attempt 1)

Playwright Test Results (private-cloud - depot-ubuntu-latest-16)

 2 passed

Details

 2 tests across 2 suites
 34 seconds
 34201ae
 🔄 Run: #17486 (attempt 1)

Playwright Test Results (private-cloud - depot-ubuntu-latest-arm-16)

 3 passed

Details

 3 tests across 3 suites
 42 seconds
 34201ae
 🔄 Run: #17486 (attempt 1)

[github-actions]

Visual Regression

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[Zaimwa9]

@gemini-code-assist review

[gemini-code-assist]

Code Review

This pull request introduces inline creation and validation of metrics within experiments, and adds an option to exclude event stats when listing warehouse connections. Feedback suggests scoping the Metric queryset in the serializer to the current environment to avoid performance issues, and making the 'exclude_event_stats' query parameter check case-insensitive for improved robustness.