Add startTimer to Gauge

Add startTimer interface to Gauge similar to how the startTimer
interface for Histogram works.

packages/lib/prometheus.js

@@ -723,6 +723,21 @@ class GaugeChild extends ValueCollectorChild {
 	setToCurrentTime() {
 		this._values.set(this._key, Date.now() / 1000);
 	}
+
+	/**
+	 * Start a timer for setting a duration for label set
+	 *
+	 * @returns {function()}
+	 *     function that when called will set the guage to the duration in
+	 *     seconds from when the timer was started
+	 */
+	startTimer() {
+		let start = process.hrtime.bigint();
+		return () => {
+			let end = process.hrtime.bigint();
+			this.set(Number(end - start) / 1e9);
+		};
+	}
 }
 
 /**
@@ -886,6 +901,18 @@ class Gauge extends ValueCollector {
 	setToCurrentTime() {
 		this._defaultChild.setToCurrentTime();
 	}
+
+	/**
+	 * Start a timer for setting a duration
+	 *
+	 * Note: Only works if this is an unlabeled collector.
+	 * @returns {function()}
+	 *     function that when called will set the guage to the duration in
+	 *     seconds from when the timer was started
+	 */
+	startTimer() {
+		return this._defaultChild.startTimer();
+	}
 }
 
 function formatValue(value) {

test/lib/prometheus.js

@@ -198,6 +198,16 @@ describe("lib/prometheus", function() {
 				);
 			});
 		});
+		describe(".startTimer()", function() {
+			it("should set the gauge to a short duration", function() {
+				let setDuration = gauge.startTimer();
+				setDuration();
+				assert(
+					gauge._values.get("") > 0 && gauge._values.get("") < 1,
+					"value set is not between 0 and 1"
+				);
+			});
+		});
 	});
 
 	describe("class Histogram", function() {