Trait Metrics 

pub trait Metrics:
    Send
    + Sync
    + 'static {
    // Provided methods
    fn on_request(&self, _method: &str) { ... }
    fn on_response(&self, _method: &str) { ... }
    fn on_error(&self, _method: &str, _error_kind: &str) { ... }
    fn on_latency(&self, _method: &str, _duration: Duration) { ... }
    fn on_queue_depth_change(&self, _active_queues: usize) { ... }
    fn on_connection_pool_stats(&self, _stats: &ConnectionPoolStats) { ... }
}

Trait for receiving metrics callbacks from the handler.

All methods have default no-op implementations so that consumers can override only the callbacks they care about.

Provided Methods

fn on_request(&self, _method: &str)

Called when a request is received, before processing.

fn on_response(&self, _method: &str)

Called when a response is successfully sent.

fn on_error(&self, _method: &str, _error_kind: &str)

Called when a request results in an error.

error_kind is a bounded, low-cardinality discriminant (e.g. ServerError::metric_label), never the free-form error message. Implementations may use it as a metric label/attribute; the caller guarantees it draws from a small fixed set, so a client cannot inflate metric cardinality through it.

fn on_latency(&self, _method: &str, _duration: Duration)

Called when a request completes (successfully or not) with the wall-clock duration from receipt to response.

This is the #1 production observability metric — use it to feed histograms, percentile trackers, or SLO dashboards.

fn on_queue_depth_change(&self, _active_queues: usize)

Called when the number of active event queues changes.

fn on_connection_pool_stats(&self, _stats: &ConnectionPoolStats)

Called with connection pool statistics when available.

Useful for monitoring connection pool health and detecting exhaustion.

Dyn Compatibility

This trait is dyn compatible.

In older versions of Rust, dyn compatibility was called "object safety".

Implementations on Foreign Types

impl<T> Metrics for Arc<T>

where T: Metrics + ?Sized,

Blanket implementation: Arc<T> implements Metrics if T does.

This eliminates the need for wrapper types like MetricsForward when sharing a metrics instance across multiple handlers or tasks.

fn on_request(&self, method: &str)

fn on_response(&self, method: &str)

fn on_error(&self, method: &str, error: &str)

fn on_latency(&self, method: &str, duration: Duration)

fn on_queue_depth_change(&self, active_queues: usize)

fn on_connection_pool_stats(&self, stats: &ConnectionPoolStats)

Implementors

impl Metrics for NoopMetrics

impl Metrics for OtelMetrics