add a metric to observe worker active time

[zyguan]

Adds a new Prometheus metric yatp_worker_active_seconds_total to observe how much time worker threads spend being active.

Summary by CodeRabbit

• New Features

  • Added worker thread activity tracking metric that measures total time worker threads spend active (not parked) in seconds, labeled by thread pool name for better performance visibility and monitoring.

• Tests

  • Added test for worker activity metric functionality.

[coderabbitai]

📝 Walkthrough

Walkthrough

These changes introduce worker activity tracking to the thread pool via a new WORKER_ACTIVE_SECONDS Prometheus metric. The metric is defined in metrics.rs, wired into the pool builder, and tracked through a WorkerActivity struct that monitors worker lifecycle events (startup, task completion, shutdown) with periodic checkpointing to flush activity data.

Changes

Cohort / File(s)	Summary
Metric Definition & Initialization src/metrics.rs, src/pool/builder.rs	Added WORKER_ACTIVE_SECONDS CounterVec metric; pool builder retrieves the metric and enables worker activity tracking by calling enable_worker_activity() on each local queue.
Worker Activity Tracking src/pool/spawn.rs	Introduced WorkerActivity struct for per-worker activity tracking; added activity field to Local<T>; implemented lifecycle methods (enable_worker_activity, on_worker_start, on_worker_end, on_task_complete) with periodic checkpointing and elapsed time accounting.
Worker Lifecycle Integration src/pool/worker.rs	Integrated lifecycle hooks into worker execution flow: calls on_worker_start() after runner initialization, on_task_complete() after each task, and on_worker_end() after draining all futures.
Test Coverage src/pool/tests.rs	Added test_worker_active_seconds_metric to verify metric collection; creates single-thread pool, executes task with brief sleep, and asserts metric counter exceeds zero.

Sequence Diagram

sequenceDiagram
    participant Pool as Thread Pool
    participant Worker as Worker Thread
    participant Local as Local<T>
    participant Activity as WorkerActivity
    participant Metric as Prometheus Counter

    Pool->>Worker: Spawn worker thread
    Worker->>Local: Initialize with activity field
    Worker->>Local: on_worker_start()
    Local->>Activity: Record start time
    Activity->>Metric: Begin tracking (if enabled)

    loop For Each Task
        Worker->>Local: Execute task
        Local->>Activity: Monitor execution
        Worker->>Local: on_task_complete()
        Local->>Activity: Check elapsed time
        alt Periodic checkpoint triggered
            Activity->>Activity: Flush accumulated time
            Activity->>Metric: Update counter with active seconds
        end
    end

    Worker->>Local: Drain remaining futures
    Worker->>Local: on_worker_end()
    Local->>Activity: Final flush
    Activity->>Metric: Commit final active seconds
    Local->>Local: Clear activity state

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Estimated code review effort

🎯 3 (Moderate) | ⏱️ ~25 minutes

Poem

🐰 Hops of joy through the threads we go,
Counting seconds as workers flow,
Active time tracked with metrics so true,
Each checkpoint a hop, each flush renewed! 🎉

🚥 Pre-merge checks | ✅ 3

✅ Passed checks (3 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title accurately describes the main change: adding a metric to track worker active time, which aligns with the introduction of WORKER_ACTIVE_SECONDS metric throughout the codebase.
Docstring Coverage	✅ Passed	Docstring coverage is 100.00% which is sufficient. The required threshold is 80.00%.

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✨ Finishing touches

• 📝 Generate docstrings

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Post copyable unit tests in a comment

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No actionable comments were generated in the recent review. 🎉

🧹 Recent nitpick comments

src/pool/spawn.rs (1)

215-220: Minor: unrelated housekeeping change.

The #[allow(dead_code)] additions for the assertion traits are reasonable (they're compile-time-only constraints), though unrelated to the metric feature. Consider splitting into a separate commit for cleaner history.

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[Copilot]

Pull request overview

Adds a new Prometheus counter metric to track how long worker threads are “active” (i.e., not parked), enabling better visibility into thread-pool utilization.

Changes:

• Introduces yatp_worker_active_seconds_total (WORKER_ACTIVE_SECONDS) labeled by pool name.
• Tracks worker active time via per-worker local counters and flushes on park/end (with periodic checkpoints while busy).
• Adds a unit test validating the metric is incremented.

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
src/metrics.rs	Defines the new WORKER_ACTIVE_SECONDS CounterVec metric.
src/pool/builder.rs	Wires a per-pool labeled counter into each worker via LocalCounter.
src/pool/spawn.rs	Implements WorkerActivity tracking and hooks into park/unpark and worker lifecycle.
src/pool/worker.rs	Calls lifecycle hooks (on_worker_start, on_task_complete, on_worker_end) from the worker loop.
src/pool/tests.rs	Adds a test ensuring the active-seconds metric increases after running work.

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[cfzjywxk]

LGTM