fix: emit warn when labels and values are out of sync

lib/guard.js

@@ -85,6 +85,9 @@ const MetricsGuard = class MetricsGuard extends stream.Transform {
 		if (labels) {
 			return Array.from(labels.keys()).map((label) => {
 				const [name, value] = label.split(SEPARATOR);
+				if (!name) {
+					this.emit("warn", "metrics", `is null ${name} ${value}`);
+				}
 				return { name, value };
 			});
 		}
@@ -131,18 +134,31 @@ const MetricsGuard = class MetricsGuard extends stream.Transform {
 			this.registry.set(metric.name, entry);
 		}
 
+		let invalidLabels = [];
 		// push all label permutations on the metric into the
 		// Set for the metric in the registry. the size of
 		// this Set is compared with the threshold
 		metric.labels.forEach((label) => {
-			entry.add(`${label.name}${SEPARATOR}${label.value}`);
+			Array.from(Object.keys(label)).forEach((key) => {
+				if (label && (typeof key === "undefined" || typeof label[key] === "undefined")) {
+					invalidLabels.push(`${metric.name} label name="${key}", value=${label[key]}`);
+				} else {
+					entry.add(`${label.name}${SEPARATOR}${label.value}`);
+				}
+			});
 		});
 
 		if (metric.source === this.id) {
 			this.emit("drop", metric);
 			next(null);
 			return;
 		}
+		if (invalidLabels.length > 0) {
+			// Omit a warning when having invalid label data
+			this.emit("warn", "labels", invalidLabels.toString());
+			next(null);
+			return;
+		}
 
 		next(null, metric);
 	}

test/guard.js

@@ -425,3 +425,50 @@ tap.test(
 		});
 	},
 );
+
+tap.test("Guard() - do not add metric when a label value is undefined and emit warning", (t) => {
+	const guard = new Guard();
+	const metric = new Metric({
+		name: "foo",
+		description: "foo",
+		labels: [{ name: "someLabel", value: undefined }],
+	});
+	const metrics = [metric];
+
+	const src = srcObjectStream(metrics);
+	const dest = destObjectStream((arr) => {
+		t.equal(arr.length, 0);
+		t.end();
+	});
+
+	guard.on("warn", (type, message) => {
+		t.equal(type, "labels");
+		t.match(message, /undefined/);
+	});
+	src.pipe(guard).pipe(dest);
+
+	setImmediate(() => {
+		dest.end();
+	});
+});
+
+tap.test("Guard() - do not add metric when label name / values are undefined and emit warning", (t) => {
+	const guard = new Guard();
+	const metrics = [new Metric({ name: "foo", description: "foo", labels: [{ name: undefined, value: null }] })];
+
+	const src = srcObjectStream(metrics);
+	const dest = destObjectStream((arr) => {
+		t.equal(arr.length, 0);
+		t.end();
+	});
+
+	guard.on("warn", (type, message) => {
+		t.equal(type, "labels");
+		t.match(message, /undefined/);
+	});
+	src.pipe(guard).pipe(dest);
+
+	setImmediate(() => {
+		dest.end();
+	});
+});