import type { IncomingMessage, ServerResponse } from "http";
/**
 * Stable string label per `MessageType` numeric enum value. Used so that
 * Prometheus labels stay human-readable across socket.io-adapter versions.
 */
export declare const MESSAGE_TYPE_NAMES: Readonly<Record<number, string>>;
export declare function messageTypeName(type: number | undefined): string;
export type RequestKind = "fetchSockets" | "broadcastWithAck" | "serverSideEmit";
/**
 * Thin abstraction over the metrics backend so the hot paths never directly
 * import `prom-client`. Implementations must be no-throw and ideally
 * zero-allocation on the no-op path.
 */
export interface MetricsRegistry {
    peerUp(uid: string): void;
    peerDown(uid: string, reason: string): void;
    expectedPeersChanged(n: number): void;
    messageSent(type: number, bytes: number): void;
    messageReceived(type: number, bytes: number): void;
    /** Start a request timer; returns a stop function (must be called exactly once). */
    requestStarted(kind: RequestKind): () => void;
    localClientsChanged(n: number): void;
    namespacesChanged(n: number): void;
    /** Build a Node http.RequestListener that serves Prometheus text format. */
    handler(): (req: IncomingMessage, res: ServerResponse) => void;
}
/**
 * Zero-cost registry used when the user does not opt into Prometheus. Methods
 * are bound to the singleton so closures and hot paths can use them safely.
 */
export declare const NoopMetricsRegistry: MetricsRegistry;
/**
 * Prometheus-backed registry. Constructing this class lazily imports
 * `prom-client`, so users who omit the `metrics` option never load it.
 */
export declare class PromMetricsRegistry implements MetricsRegistry {
    private readonly registry;
    private readonly client;
    private readonly gPeersConnected;
    private readonly gPeersExpected;
    private readonly cMessagesSent;
    private readonly cMessagesReceived;
    private readonly hMessageBytes;
    private readonly hRequestDuration;
    private readonly cPeerDrops;
    private readonly gNamespaces;
    private readonly gIoClients;
    constructor(opts?: {
        defaultLabels?: Record<string, string>;
    });
    peerUp(_uid: string): void;
    peerDown(_uid: string, reason: string): void;
    expectedPeersChanged(n: number): void;
    messageSent(type: number, bytes: number): void;
    messageReceived(type: number, bytes: number): void;
    requestStarted(kind: RequestKind): () => void;
    localClientsChanged(n: number): void;
    namespacesChanged(n: number): void;
    handler(): (req: IncomingMessage, res: ServerResponse) => void;
}
//# sourceMappingURL=metrics.d.ts.map