Fix Cluster and Replica Set labels in Metrics (#394)

## Issue
Cluster and replica set labels in Grafana showed None

## Solution
Update how we scrape those configs

---------

Co-authored-by: Ubuntu <ubuntu@ip-172-31-17-108.eu-central-1.compute.internal>

src/charm.py

@@ -137,15 +137,30 @@ def __init__(self, *args):
         # relation events for Prometheus metrics are handled in the MetricsEndpointProvider
         self._grafana_agent = COSAgentProvider(
             self,
-            metrics_endpoints=Config.Monitoring.METRICS_ENDPOINTS,
             metrics_rules_dir=Config.Monitoring.METRICS_RULES_DIR,
             logs_rules_dir=Config.Monitoring.LOGS_RULES_DIR,
             log_slots=Config.Monitoring.LOG_SLOTS,
+            scrape_configs=self._mongo_scrape_config,
         )
 
         self.secrets = SecretCache(self)
 
     # BEGIN: properties
+    def _mongo_scrape_config(self) -> List[Dict]:
+        """Generates scrape config for the mongo metrics endpoint."""
+        return [
+            {
+                "metrics_path": "/metrics",
+                "static_configs": [
+                    {
+                        "targets": [
+                            f"{self._unit_ip(self.unit)}:{Config.Monitoring.MONGODB_EXPORTER_PORT}"
+                        ],
+                        "labels": {"cluster": self.app.name, "replication_set": self.app.name},
+                    }
+                ],
+            }
+        ]
 
     @property
     def primary(self) -> str:

src/grafana_dashboards/MongoDB_ReplSet_Summary.json

@@ -2810,7 +2810,7 @@
                 "label": "Replica Set",
                 "multi": false,
                 "multiFormat": "glob",
-                "name": "replset",
+                "name": "replication_set",
                 "options": [],
                 "query": {
                     "query": "label_values(mongodb_mongod_replset_my_state{cluster=~\"$cluster\"}, set)",