feat: Enable OpenTelemetry support

[khvn26]

• 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

Contributes to Flagsmith/flagsmith-common#182.

Bumps flagsmith-common from 3.5.0 to 3.6.0, which includes OTel instrumentation support.

Adds traceparent, tracestate, and baggage to the default FLAGSMITH_CORS_EXTRA_ALLOW_HEADERS so browser-based SDKs can forward W3C distributed trace context to the API.

How did you test this code?

Verified the lock file resolves cleanly and pre-commit hooks pass. The new OTel setup is opt-in, so no behaviour change without configuration.

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3 Skipped Deployments

Project	Deployment	Actions	Updated (UTC)
docs	Ignored		Apr 3, 2026 5:58pm
flagsmith-frontend-preview	Ignored		Apr 3, 2026 5:58pm
flagsmith-frontend-staging	Ignored		Apr 3, 2026 5:58pm

[github-actions]

Docker builds report

Image	Build Status	Security report
ghcr.io/flagsmith/flagsmith-e2e:pr-7132	Finished ✅	Skipped
ghcr.io/flagsmith/flagsmith-api-test:pr-7132	Finished ✅	Skipped
ghcr.io/flagsmith/flagsmith-api:pr-7132	Finished ✅	Results ✅
ghcr.io/flagsmith/flagsmith:pr-7132	Finished ✅	Results ✅
ghcr.io/flagsmith/flagsmith-private-cloud:pr-7132	Finished ✅	Results ✅
ghcr.io/flagsmith/flagsmith-frontend:pr-7132	Finished ✅	Results ✅

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.26%. Comparing base (7f8e104) to head (865f3e5).
⚠️ Report is 4 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #7132      +/-   ##
==========================================
- Coverage   98.34%   98.26%   -0.09%     
==========================================
  Files        1336     1336              
  Lines       50128    49966     -162     
==========================================
- Hits        49298    49098     -200     
- Misses        830      868      +38     

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[github-actions]

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

 10 passed

Details

 10 tests across 7 suites
 24.6 seconds
 865f3e5
 🔄 Run: #15711 (attempt 1)

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

 10 passed

Details

 10 tests across 7 suites
 46.9 seconds
 865f3e5
 🔄 Run: #15711 (attempt 1)

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

 2 passed

Details

 2 tests across 2 suites
 47.7 seconds
 865f3e5
 🔄 Run: #15711 (attempt 1)

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

 2 passed

Details

 2 tests across 2 suites
 1 minute, 6 seconds
 865f3e5
 🔄 Run: #15711 (attempt 1)