Update to prom-client 15.x

Change-type: minor

Author: Page-

package.json

@@ -24,7 +24,7 @@
     "debug": "^4.3.4",
     "express": "^4.17.3",
     "on-finished": "^2.4.1",
-    "prom-client": "^14.0.1",
+    "prom-client": "^15.1.3",
     "typed-error": "^3.2.1"
   },
   "devDependencies": {