Switch to performance.now() for timing calculations.

Better browser support. #685

Author: jdmarshall

example/gauge.js

@@ -13,6 +13,8 @@ async function main() {
 		labelNames: ['code'],
 	});
 
+	const done = g.startTimer({});
+
 	g.set({ code: 200 }, 5);
 	console.log(await register.metrics());
 	/*
@@ -49,6 +51,8 @@ async function main() {
 	*/
 
 	g.set(22);
+	done();
+
 	console.log(await register.metrics());
 	/*
 	# HELP test_gauge Example of a gauge

lib/gauge.js

@@ -86,12 +86,12 @@ class Gauge extends Metric {
 	 * });
 	 */
 	startTimer(labels) {
-		const start = process.hrtime();
+		const start = performance.now();
+
 		return endLabels => {
-			const delta = process.hrtime(start);
-			const value = delta[0] + delta[1] / 1e9;
-			this.set(Object.assign({}, labels, endLabels), value);
-			return value;
+			const delta = (performance.now() - start) / 1e3;
+			this.set(Object.assign({}, labels, endLabels), delta);
+			return delta;
 		};
 	}