feat: upgrade prom-client from 12.0.0 to 15.1.3

Snyk has created this PR to upgrade prom-client from 12.0.0 to 15.1.3.

See this package in npm:
prom-client

See this project in Snyk:
https://app.snyk.io/org/doperiddle/project/09cea5ed-d730-4815-9858-2b92e1f152c3?utm_source=github&utm_medium=referral&page=upgrade-pr

package.json

@@ -48,7 +48,7 @@
     "knex": "^0.21.5",
     "knex-stringcase": "^1.4.5",
     "pg": "8.2.1",
-    "prom-client": "^12.0.0",
+    "prom-client": "^15.1.3",
     "semver": "^7.3.2",
     "typescript": "^3.9.5"
   },