/**
* Prometheus-style counters + histograms for `email.send`. Exposes a
* `.expose()` helper that produces plaintext in the Prometheus
* exposition format — hand this to a `/metrics` endpoint in any
* framework.
*
* OTLP export is covered by the existing `withTelemetry` middleware.
*
* @module
*/
import type { Middleware } from "../types.mjs";
export interface MetricsRegistry {
	readonly sends: number;
	readonly errors: Record<string, number>;
	readonly durationsMs: number[];
	incSend: (driver: string) => void;
	incError: (driver: string, code: string) => void;
	observeDuration: (driver: string, ms: number) => void;
	expose: () => string;
}
export declare function createMetricsRegistry(): MetricsRegistry;
export interface MetricsMiddlewareOptions {
	registry: MetricsRegistry;
	now?: () => number;
}
/** Middleware that records counters + durations into a registry. */
export declare function withMetrics(options: MetricsMiddlewareOptions): Middleware;