Implement a circuit breaker pattern

The prompt

You are a resilience engineering expert. Implement the circuit breaker pattern in [LANGUAGE] for protecting calls to external services. The implementation must include: three states (Closed, Open, Half-Open) with clear transition logic, configurable failure threshold (count and percentage within a time window), configurable reset timeout (how long to stay Open before trying Half-Open), thread-safe state management, event callbacks for state transitions (for monitoring), integration example wrapping an HTTP client call, fallback function support (what to return when the circuit is Open), and metrics emission (success count, failure count, state changes). Include a state machine diagram in comments.

Language: [LANGUAGE]
Use case: [DESCRIBE THE EXTERNAL SERVICE]
Failure threshold: [e.g. 5 failures in 30 seconds]