ci(e2e): Remove app metrics job and performance-tests

[antonis]

📢 Type of change

• Bugfix
• New feature
• Enhancement
• Refactoring

📜 Description

Removes the metrics job (getsentry/action-app-sdk-overhead-metrics) from e2e-v2.yml and the performance-tests/ apps it built (TestAppPlain, TestAppSentry, metrics configs, fastlane setup).

Cleans up the leftover references:

• package.json workspaces, lerna.json, yarn.lock
• .swiftlint.yml, scripts/google-java-format.sh, scripts/pmd.sh
• perf_tests filter in .github/file-filters.yml and output in detect-changes.yml
• CONTRIBUTING.md

💡 Motivation and Context

Closes #6890. Same move as sentry-cocoa (getsentry/sentry-cocoa#9350) and sentry-dart (getsentry/sentry-dart#4091).

• Cost: 4 matrix jobs (2 on macos-26-xlarge) at ~12–14 min each, serialized by max-parallel: 2 for Sauce Labs, plus 50 Sauce Labs startup runs per job.
• Size: now gated by Sentry Size Analysis (size-analysis.yml). Status check and PR comment rules are configured for sentry-react-native (absolute diff > 50 KB on install and download size, iOS + Android). Verified in test: Verify Size Analysis threshold (do not merge) #6895: a +480 KB asset failed the Size Analysis check and posted a PR comment, while a small change in feat(tracing): Connect React Native traces with embedded WebViews #6794 passed.
• Startup: the startup-time gate is dropped with no replacement. Its thresholds (-3000..+150 ms on iOS) were too loose and noisy over cloud devices to catch real regressions.

The metrics job was not a required status check.

💚 How did you test it?

• YAML of edited workflows/configs parses; yarn install updates yarn.lock by only dropping the two TestApp workspaces.
• No remaining references to performance-tests, perf_tests, TestAppPlain or TestAppSentry.
• CI on this PR.

📝 Checklist

• I added tests to verify changes.
• No new PII added or SDK only sends newly added PII if sendDefaultPII is enabled.
• I updated the docs if needed.
• I updated the wizard if needed.
• All tests passing.
• Public API changes reviewed by another Mobile SDK team member or implemented according to the develop docs spec.
• No breaking changes.

🔮 Next steps

• SAUCE_USERNAME / SAUCE_ACCESS_KEY are no longer referenced by any workflow; they can be removed from repo secrets if nothing else needs them.

🤖 Generated with Claude Code

[github-actions]

Semver Impact of This PR

⚪ None (no version bump detected)

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• ci(e2e): Remove app metrics job and performance-tests by antonis in #6896

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