Refactor Timing and TimingCollector classes; update README for improved usage instructions

Author: sergiodxa

README.md

@@ -44,13 +44,13 @@ You can also manually collect timings:
 ```ts
 import { Timing } from "@edgefirst-dev/server-timing";
 
+// measures are taken from the time this is created
 let timing = new Timing("name", "description");
 
-timing.measure(async () => {
-  // do something
-});
+await doSomething(); // do something
 
-collector.add(timing);
+timing.end(); // end the measurement
+collector.add(timing); // add the timing to the collector
 ```
 
 Each `Timing` can be used once. If you want to take different measurements, create a new `Timing` instance.

src/lib/timing-collector.ts

@@ -36,7 +36,10 @@ export class TimingCollector {
 			typeof descriptionOrFn === "string" ? descriptionOrFn : undefined,
 		);
 
-		return timing.measure(callback).finally(() => void this.collect(timing));
+		return callback().finally(() => {
+			timing.end();
+			this.collect(timing);
+		});
 	}
 
 	/**

src/lib/timing.ts

@@ -1,26 +1,21 @@
 export class Timing implements PerformanceServerTiming {
-	start = 0;
-	end = 0;
+	readonly #start = performance.now();
+	#end = 0;
 
 	constructor(
 		public name: string,
 		public description = "",
 	) {}
 
 	get duration() {
-		if (this.end === 0) return 0;
-		let result = this.end - this.start;
+		if (this.#end === 0) return 0;
+		let result = this.#end - this.#start;
 		if (result < 0) return 0;
 		return result;
 	}
 
-	async measure<T>(fn: Timing.MeasureFunction<T>): Promise<T> {
-		this.start = performance.now();
-		try {
-			return await fn();
-		} finally {
-			this.end = performance.now();
-		}
+	end() {
+		this.#end = performance.now();
 	}
 
 	toString() {