Fix metrics broken by comma and equal in labels

The logic for turning a key back into the labelles that defined it would
not account for the labels values possibly including a comma or equal
sign, which in turn broke the logic for handling these metrics.

Fix by properly parsing the keys.

test/lib/prometheus.js

@@ -651,8 +651,21 @@ describe("lib/prometheus", function() {
 				let gauge = new prometheus.Gauge(
 					"test", "Help", { register: false, labels: ["a", "b"] }
 				);
-				gauge.labels('x\\\\\n\"\"\n\\', "2");
-				gauge.removeAll({ a: 'x\\\\\n\"\"\n\\' });
+
+				let labelValue = "";
+				for (let i = 0; i < 128; i++) {
+					const char = String.fromCodePoint(i);
+					if (!/[1-9B-Zb-z]/.test(char)) { // exclude boring characters
+						labelValue += char;
+					}
+				}
+				labelValue += "©💣";
+
+				gauge.labels(labelValue, "2");
+				gauge.removeAll({ a: labelValue });
+				assert(gauge._values.size === 0, "Matching labels remained");
+				gauge.labels("1", labelValue);
+				gauge.removeAll({ a: "1" });
 				assert(gauge._values.size === 0, "Matching labels remained");
 			});
 		});