feat: implemented metrics feature to track workers

[fahimfaisaal]

• Added metrics state inside worker to track tasks
• Included new method NumPending with worker to provide the number of pending tasks
• Added Prometheus example to demonstrate how to integrate varmq

[coderabbitai]

Walkthrough

Adds a Metrics abstraction and wires per-job metrics (submitted, completed, successful, failed) across workers and queues; extends Worker with Metrics() and NumPending(); updates constructors to record success/failure; provides tests, a Prometheus example, and enables the race detector for tests.

Changes

Cohort / File(s)	Summary
Metrics core metrics.go, metrics_test.go	New exported Metrics interface and internal metrics implementation with atomic counters, increments, Reset, and concurrent-safety tests.
Worker surface & behavior worker.go, main.go, worker_test.go, main_test.go	Add metrics Metrics field; implement Metrics() Metrics and NumPending() int on Worker; refactor constructors to capture worker instance and increment success/failure/completed metrics; add tests for NumPending and error-worker success.
Queue instrumentation queue.go, persistent.go, persistent_priority.go, priority.go	Call q.w.Metrics().incSubmitted() after successful Enqueue (Add/AddAll) across queue variants to account submissions.
Examples & modules examples/prometheus/main.go, examples/prometheus/go.mod	Add Prometheus example demonstrating metric registration and /metrics endpoint; new module file for the example with local replace.
Build/test Makefile, go.mod	Enable -race flag in test target; bump github.com/stretchr/testify to v1.11.1.

Sequence Diagram(s)

sequenceDiagram
    participant Client
    participant Queue
    participant Worker
    participant Metrics
    participant JobProcessor

    Client->>Queue: Add(job)
    Queue->>Metrics: incSubmitted()
    Metrics->>Metrics: submitted++ 

    Queue->>Worker: notify / wake

    Worker->>JobProcessor: dequeue & execute
    JobProcessor->>JobProcessor: run job
    alt success
        JobProcessor->>Metrics: incSuccessful()
        Metrics->>Metrics: successful++
    else failure
        JobProcessor->>Metrics: incFailed()
        Metrics->>Metrics: failed++
    end
    JobProcessor->>Metrics: incCompleted()
    Metrics->>Metrics: completed++
    
    Client->>Worker: Metrics()
    Worker-->>Client: Metrics instance

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Estimated code review effort

🎯 4 (Complex) | ⏱️ ~45 minutes

Possibly related PRs

• refactor: update worker implementation to use new pool structure #47 — touches worker.go constructors and pool fields; likely overlaps with the worker refactor and metrics wiring.
• feat: implemented multi queue bound mechanism with single worker #37 — modifies core queue/worker files (queue.go, worker.go, persistent files); related to enqueue and worker initialization changes.
• refactor: simplify worker by passing job interface instead only data #27 — updates NewWorker/NewErrWorker/NewResultWorker constructors and panic-safe wrappers; intersects with constructor refactors here.

Poem

🐰 I counted hops from queue to beeps and bells,
Submitted, done, and failed — where each counter dwells.
Workers hum softly, metrics shining bright,
Prometheus listens through day and night.
A rabbit's small cheer for observability delight!

🚥 Pre-merge checks | ✅ 1 | ❌ 2

❌ Failed checks (2 warnings)

Check name	Status	Explanation	Resolution
Description check	⚠️ Warning	The PR description is incomplete relative to the template. It lacks the required sections (Type of Change, Changes Made, Testing, Performance Impact, Checklist, Related Issues) and instead provides only a brief bullet-point summary without structural organization.	Fill in all required template sections: explicitly mark Type of Change as 'New feature', detail the specific changes made, confirm tests pass and describe new tests added, assess performance impact, and complete the checklist items. Include any related issue numbers.
Docstring Coverage	⚠️ Warning	Docstring coverage is 53.85% which is insufficient. The required threshold is 80.00%.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (1 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title accurately describes the main change: implementing a metrics feature to track workers. It is concise, specific, and clearly summarizes the primary objective of the changeset.

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✨ Finishing touches

• 📝 Generate docstrings

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Post copyable unit tests in a comment
• Commit unit tests in branch feat/add-metrics

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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