Allow metrics hook handlers to return nothing

A handler that only updates gauges has nothing to yield. collect() already
dropped undefined results at runtime, now its type says so too.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

packages/controller/src/BaseControllerPlugin.ts

@@ -125,7 +125,7 @@ export class ControllerHooks extends lib.AsyncHookCollection {
 	 *
 	 * @returns an async iterator of prometheus metric results or undefined.
 	 */
-	readonly metrics: lib.AsyncHook<[], AsyncIterable<lib.CollectorResult>>;
+	readonly metrics: lib.AsyncHook<[], AsyncIterable<lib.CollectorResult> | void>;
 
 	/**
 	 * Called when the controller is shutting down

packages/host/src/BaseHostPlugin.ts

@@ -46,7 +46,7 @@ export class HostHooks extends lib.AsyncHookCollection {
 	 *
 	 * @returns an async iterator of prometheus metric results or undefined.
 	 */
-	readonly metrics: lib.AsyncHook<[], AsyncIterable<lib.CollectorResult>>;
+	readonly metrics: lib.AsyncHook<[], AsyncIterable<lib.CollectorResult> | void>;
 
 	/**
 	 * Called when the host is shutting down

packages/host/src/BaseInstancePlugin.ts

@@ -52,7 +52,7 @@ export class InstanceHooks extends lib.AsyncHookCollection {
 	 *
 	 * @returns an async iterator of prometheus metric results or undefined.
 	 */
-	readonly metrics: lib.AsyncHook<[], AsyncIterable<lib.CollectorResult>>;
+	readonly metrics: lib.AsyncHook<[], AsyncIterable<lib.CollectorResult> | void>;
 
 	/**
 	 * Called after the Factorio server is started

packages/lib/src/AsyncHook.ts

@@ -111,7 +111,7 @@ export class AsyncHook<
 	 *
 	 * Returns all successful results.
 	 */
-	async collect(...args: Args): Promise<Result[]> {
+	async collect(...args: Args): Promise<Exclude<Awaited<Result>, void>[]> {
 		const results = await Promise.all(
 			[...this._handlers.entries()].map(
 				([name, handler]) => this._invokeHandler(name, handler, args)
@@ -119,7 +119,7 @@ export class AsyncHook<
 		);
 
 		return results.filter(
-			(result): result is Awaited<Result> => result !== undefined
+			(result): result is Exclude<Awaited<Result>, void> => result !== undefined
 		);
 	}