Add prom-client group to package rules (#215)

Author: phnx47-bot

.github/renovate.json

@@ -8,6 +8,11 @@
   "commitMessageTopic": "{{depName}}",
   "ignoreDeps": ["Microsoft.SourceLink.GitHub", "StyleCop.Analyzers"],
   "packageRules": [
+    {
+      "automerge": true,
+      "groupName": "prom-client",
+      "matchSourceUrls": ["https://github.com/prom-client-net{/,}**"]
+    },
     {
       "automerge": true,
       "groupName": "coverlet packages",