Add Prometheus metrics

Closes #197

Co-authored-by: haywo <haywo@nct08.de>

example-config.yaml

@@ -49,6 +49,12 @@ appservice:
     as_token: "This value is generated when generating the registration"
     hs_token: "This value is generated when generating the registration"
 
+metrics:
+    # Whether or not to enable prometheus metrics
+    enabled: false
+    # IP and port where the metrics listener should be. The path is always /metrics
+    listen: 127.0.0.1:8001
+
 # Bridge config
 bridge:
     # Localpart template of MXIDs for WhatsApp users.