dispatcher: add suppressed alerts metric by reason

[ashleyprimo]

Add alertmanager_alerts_suppressed, a gauge for active non-resolved alerts suppressed by reason (silence or inhibition); intention being able to ascertain how many alerts are currently not firing due to a silence (useful for anomaly detection, for example a silence that covers large blast radius)

Pull Request Checklist

Please check all the applicable boxes.

• Is this a new feature?
  • I have added tests that test the new feature's functionality
• Is this a breaking change?
  • My changes do not break the existing cluster messages
  • My changes do not break the existing api
• I have added/updated the required documentation
• I have signed-off my commits
• I will follow best practices for contributing to this project

Which user-facing changes does this PR introduce?

  [FEATURE] dispatcher: Add `alertmanager_alerts_suppressed` metric for active alerts suppressed by silence or inhibition.

Summary by CodeRabbit

• New Features

  • Added alertmanager_alerts_suppressed Prometheus metric to count alerts suppressed by silence or inhibition
  • Metrics can include an optional group_key label when group-key metrics are enabled
  • Optional alertname label support for relevant metrics when enabled

• Tests

  • Added tests covering suppressed-alerts metric emission with and without group_key

• Documentation

  • Updated metrics docs to describe the new metric and label behavior

[coderabbitai]

📝 Walkthrough

Walkthrough

Adds a new Prometheus metric alertmanager_alerts_suppressed counting alerts suppressed by reason (silence or inhibition), implements collector descriptor/storage and dual collection paths for group-key-enabled and disabled modes, updates emission helpers and counting model, and adds tests and documentation.

Changes

Suppressed Alert Metrics

Layer / File(s)	Summary
Metric descriptor and collector setup dispatch/metric.go	New alertmanager_alerts_suppressed descriptor is created with state and reason labels, optionally including group_key. Collector stores and describes both the existing state metric and the new suppressed metric.
Counting model and helper methods dispatch/metric.go	New types alertCounts and alertStatusSnapshot model state and suppression-reason tracking. Methods countAlerts, addState, addSuppressedReasons accumulate counts; emitCounts emits per-state and suppressed-by-reason metrics. emit now accepts a descriptor parameter.
Collection with group-key metrics enabled dispatch/metric.go	When group-key metrics are enabled, Collect calls emitCounts per aggregation group with the group-key label, emitting both state and suppressed-by-reason counts grouped by aggregation key.
Collection with group-key metrics disabled dispatch/metric.go	When group-key metrics are disabled, deduplication is rewritten to track per-fingerprint highest-priority state plus silenced/inhibited flags via snapshots, then totals are computed and emitted without group labels.
Suppressed metrics test suite dispatch/metric_test.go	Two tests validate the suppressed-by-reason metric: one without group-key labels and one with group-key labels enabled. Helper constructors build test dispatchers, routes, aggregation groups, and alerts with silenced/inhibited states.
Metric documentation updates docs/alertmanager.md	Documentation for alertmanager_alerts_suppressed describes suppression reasons and group_key label support. Notes added about alertname label when alert-names-in-metrics is enabled.

🎯 3 (Moderate) | ⏱️ ~25 minutes

Suggested reviewers:

• SoloJacobs
• siavashs

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 14.29% which is insufficient. The required threshold is 80.00%.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Description check	✅ Passed	The PR description is complete: it explains the feature, includes checked items for tests and documentation, documents no breaking changes, and provides release notes with the [FEATURE] prefix.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Title check	✅ Passed	The title accurately summarizes the main change: adding a new suppressed alerts metric that breaks down counts by reason (silence vs inhibition).

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✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests

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[ashleyprimo]

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[ashleyprimo]

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