[chore] Add extra labels to spec observability metrics

[Horiodino]

Description:
Added support for extraLabels in spec.observability.metrics to allow users to attach additional labels to the generated ServiceMonitor.

Fixes: #4138 (comment)

Link to tracking Issue(s):

• Resolves: Add extraLabels to spec.observability.metrics #4138

Testing:

• Added unit tests to cover cases:
  • With empty extraLabels
  • With custom extraLabels map containing multiple labels
• Verified generated ServiceMonitor contains the expected labels.

Documentation:

• Updated API docs to document the new extraLabels field under spec.observability.metrics.
• Regenerated CRDs and bundle manifests to include the new field.

[iblancasa]

Please add:

• a changelog
• improve the e2e test to check this new feature

[github-actions]

E2E Test Results

 33 files  ±0  221 suites  ±0   3h 36m 13s ⏱️ - 9m 30s
 84 tests ±0   84 ✅ +1  0 💤 ±0  0 ❌  - 1 
221 runs  ±0  221 ✅ +1  0 💤 ±0  0 ❌  - 1 

Results for commit ce341ea. ± Comparison against base commit ad9d30a.

♻️ This comment has been updated with latest results.

[Horiodino]

@iblancasa could you please cross-check #4184?
I thought it might be related to the changes from this PR, but I already tested it locally and didn’t get any errors:

✓  internal/instrumentation (21.838s)
✓  internal/controllers (40.73s)

=== Skipped
=== SKIP: internal/controllers TestRegisterWithManager_OpAMPBridge (0.00s)
    opampbridge_controller_test.go:159: this test requires a real cluster, otherwise the GetConfigOrDie will die

=== SKIP: internal/controllers TestRegisterWithManager (0.00s)
    reconcile_test.go:1188: this test requires a real cluster, otherwise the GetConfigOrDie will die

DONE 1795 tests, 2 skipped in 55.687s

[swiatekm]

@Horiodino rebasing on main should fix that flaky test.

The objections have been addressed, and the reviewer is currently unavailable. Dismissing to avoid blocking the PR unnecessarily.

[swiatekm]

Thank you for the contribution and patience with the review process, @Horiodino!

[danielfm]

Will this change be bundled into the next stable release? Any expected date for the launch?