Document unreachable code paths in keyToLabels

These were added to as a diagnostic in case the logic turns out to be
broken.  They should not be possible to reach as any value produced by
labelsToKeys will not cause the error conditions to occur.

packages/lib/src/prometheus.ts

@@ -276,13 +276,16 @@ function keyToLabels(key: string) {
 	let matchLabel = /([^=]+)="(([^\\"]|\\["n\\])+)"/y;
 	while (pos < key.length) {
 		if (pos !== 0) {
+			/* istanbul ignore if */
 			if (key[pos] !== ",") {
+				// Should be unreachable
 				throw Error(`Expected , at pos ${pos} in "${key}"`);
 			}
 			pos += 1;
 		}
 		matchLabel.lastIndex = pos;
 		const match = matchLabel.exec(key);
+		/* istanbul ignore else */
 		if (match) {
 			let [, name, value] = match;
 			labels.set(name, value
@@ -292,6 +295,7 @@ function keyToLabels(key: string) {
 			);
 			pos += match[0].length;
 		} else {
+			// Should be unreachable
 			throw new Error(`Failed to parse label at pos ${pos} in "${key}"`);
 		}
 	}