Metrics Collector Generator

Creates metrics collection infrastructure for application observability and monitoring.

When to Use

Scenario	Example
Request monitoring	Track request rate, errors, duration (RED)
Business metrics	Count orders, revenue, active users
Performance profiling	Histogram of response times
Health dashboards	Expose /metrics for Prometheus scraping

Component Characteristics

MetricsCollectorInterface

• Metrics contract for increment, gauge, histogram operations
• Backend-agnostic: swap Prometheus for StatsD or custom
• Supports labels for dimensional metrics

Counter

• Monotonically increasing metric wrapper
• Tracks totals: requests, errors, processed items
• Supports labels for per-endpoint counts

Gauge

• Point-in-time value metric wrapper
• Tracks current state: active connections, queue size
• Can increase and decrease

Histogram

• Distribution metric wrapper
• Tracks request duration, response size
• Configurable bucket boundaries

PrometheusMetricsCollector

• Prometheus PHP client adapter
• Registers counters, gauges, histograms
• Thread-safe with shared storage

MetricsMiddleware

• PSR-15 middleware collecting RED metrics automatically
• Rate: total request count per endpoint
• Errors: error count by status code
• Duration: response time histogram

MetricsAction

• /metrics endpoint exposing Prometheus format
• Returns text/plain with all registered metrics

NullMetricsCollector

• Null Object pattern for testing and development
• All operations are no-ops
• Drop-in replacement without side effects

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Generation Process

Step 1: Generate Core Components

Path: src/Infrastructure/Metrics/

1. MetricsCollectorInterface.php — Metrics contract
2. Counter.php — Counter metric wrapper
3. Gauge.php — Gauge metric wrapper
4. Histogram.php — Histogram metric wrapper

Step 2: Generate Implementations

Path: src/Infrastructure/Metrics/

1. PrometheusMetricsCollector.php — Prometheus adapter
2. NullMetricsCollector.php — Null Object for testing

Step 3: Generate HTTP Components

Path: src/Infrastructure/Metrics/

1. MetricsMiddleware.php — PSR-15 RED metrics middleware
2. MetricsAction.php — /metrics endpoint action

Step 4: Generate Tests

1. NullMetricsCollectorTest.php — Null object behavior tests
2. MetricsMiddlewareTest.php — Middleware metrics tests

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File Placement

Component	Path
All Classes	src/Infrastructure/Metrics/
Unit Tests	tests/Unit/Infrastructure/Metrics/

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Naming Conventions

Component	Pattern	Example
Interface	MetricsCollectorInterface	MetricsCollectorInterface
Counter	Counter	Counter
Gauge	Gauge	Gauge
Histogram	Histogram	Histogram
Prometheus	PrometheusMetricsCollector	PrometheusMetricsCollector
Null Object	NullMetricsCollector	NullMetricsCollector
Middleware	MetricsMiddleware	MetricsMiddleware
Action	MetricsAction	MetricsAction
Test	{ClassName}Test	MetricsMiddlewareTest

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Quick Template Reference

MetricsCollectorInterface

interface MetricsCollectorInterface
{
    /** @param array<string, string> $labels */
    public function increment(string $name, array $labels = [], float $value = 1.0): void;

    /** @param array<string, string> $labels */
    public function gauge(string $name, float $value, array $labels = []): void;

    /** @param array<string, string> $labels */
    public function histogram(string $name, float $value, array $labels = []): void;
}

MetricsMiddleware

final readonly class MetricsMiddleware implements MiddlewareInterface
{
    public function process(
        ServerRequestInterface $request,
        RequestHandlerInterface $handler
    ): ResponseInterface;
}

NullMetricsCollector

final readonly class NullMetricsCollector implements MetricsCollectorInterface
{
    public function increment(string $name, array $labels = [], float $value = 1.0): void {}
    public function gauge(string $name, float $value, array $labels = []): void {}
    public function histogram(string $name, float $value, array $labels = []): void {}
}

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Usage Example

// Middleware auto-collects RED metrics
$middleware = new MetricsMiddleware($collector);

// Manual metrics in use cases
$collector->increment('orders_created_total', ['channel' => 'web']);
$collector->gauge('active_connections', $pool->activeCount());
$collector->histogram('payment_duration_seconds', $elapsed, ['gateway' => 'stripe']);

// Expose via /metrics endpoint
$app->get('/metrics', new MetricsAction($collector));

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RED Metrics Pattern

Rate     ──→ http_requests_total{method, path, status}
Errors   ──→ http_requests_total{status=~"5.."}
Duration ──→ http_request_duration_seconds{method, path}

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Anti-patterns to Avoid

Anti-pattern	Problem	Solution
High cardinality labels	Prometheus storage explosion	Limit label values, no user IDs
No null implementation	Cannot disable metrics in tests	NullMetricsCollector
Inline metric names	Typos, inconsistency	Define constants or enum
Missing error metrics	Cannot calculate error rate	Track status codes
No histogram buckets	Default buckets don't fit use case	Configure per metric
Metrics in domain layer	Infrastructure leak	Keep in infrastructure only

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References

For complete PHP templates and examples, see:

• references/templates.md — MetricsCollectorInterface, Counter, Gauge, Histogram, Prometheus, Middleware templates
• references/examples.md — Integration examples and tests