Add Metrics Hooks #12

Developers often want visibility into retry behavior and circuit breaker state transitions for monitoring and analytics.

Add optional metrics hooks that allow consumers to collect operational data without requiring a full observability platform.

Potential use cases:

• Prometheus metrics
• OpenTelemetry exporters
• Internal monitoring dashboards
• Custom analytics systems

Example

createResilientFetch({
onRetry: () => metrics.increment("retries"),
onFailure: () => metrics.increment("failures"),
onCircuitOpen: () => metrics.increment("circuit_open")
});

Acceptance Criteria

• Design metrics hook interface
• Support retry metrics
• Support failure metrics
• Support circuit breaker metrics
• Add tests
• Update documentation