feat: test execution metrics collection

[thomhurst]

Closes #4896. Adds --metrics flag with TestMetricsCollector.

[claude]

Code Review

Bug: Asymmetric concurrency counter for dependency-skipped tests

The --metrics flag feature is well-designed overall — the use of Interlocked operations for thread safety, ValueTask for async methods, and the zero-overhead opt-in via nullable _metricsCollector are all solid choices.

However, there is a logic bug in how OnTestCompleted is called for dependency-skipped tests in TestRunner.cs.

The problem:

In TestRunner.cs lines 85–90, when a test is skipped due to a failed dependency, OnTestCompleted is called without a prior call to OnTestStarted:

TUnit/TUnit.Engine/Scheduling/TestRunner.cs

Lines 84 to 92 in b95e529

	if (dependency.Test.State != TestState.Passed && !dependency.ProceedOnFailure)
	{
	_testStateManager.MarkSkipped(test, "Skipped due to failed dependencies");
	_metricsCollector?.OnTestCompleted(passed: false, skipped: true, duration: null);
	await _tunitMessageBus.Skipped(test.Context, "Skipped due to failed dependencies").ConfigureAwait(false);
	return;
	}
	}

OnTestStarted (which increments _currentConcurrentTests) is only called at line 97, which is after the early return on line 90:

TUnit/TUnit.Engine/Scheduling/TestRunner.cs

Lines 95 to 100 in b95e529

	test.StartTime = DateTimeOffset.UtcNow;
	_metricsCollector?.OnTestStarted();
	try
	{

Meanwhile, OnTestCompleted unconditionally decrements _currentConcurrentTests as its very first operation:

TUnit/TUnit.Engine/Services/TestMetricsCollector.cs

Lines 68 to 72 in b95e529

	public void OnTestCompleted(bool passed, bool skipped, TimeSpan? duration)
	{
	Interlocked.Decrement(ref _currentConcurrentTests);
	if (skipped)

The consequence:

Every dependency-skipped test decrements _currentConcurrentTests without a matching increment, driving it negative. In a test suite with N dependency-skipped tests, the counter starts at -N before any real tests run. When 8 tests then run concurrently, _currentConcurrentTests only reaches 8 - N, so _peakConcurrentTests is reported as 8 - N instead of 8. The "Peak concurrent tests" metric in the summary is definitively wrong.

Suggested fix:

The cleanest fix is to add a dedicated OnTestSkippedBeforeStart method to TestMetricsCollector that only increments the skip counter without touching _currentConcurrentTests:

// In TestMetricsCollector.cs
public void OnTestSkippedBeforeStart()
{
    Interlocked.Increment(ref _totalTests);
    Interlocked.Increment(ref _skippedTests);
}

And in TestRunner.cs, replace line 88:

// Before:
_metricsCollector?.OnTestCompleted(passed: false, skipped: true, duration: null);

// After:
_metricsCollector?.OnTestSkippedBeforeStart();

This separates the "test lifecycle" tracking (_currentConcurrentTests) from the "test result" tracking (skip/pass/fail counts), which is a cleaner abstraction since a dependency-skipped test never actually started from the concurrency perspective.