Only insert labels that do not already exist (#353)

lib/charms/loki_k8s/v1/loki_push_api.py

@@ -518,7 +518,7 @@ def _alert_rules_error(self, event):
 
 # Increment this PATCH version before using `charmcraft publish-lib` or reset
 # to 0 if you are raising the major API version
-LIBPATCH = 3
+LIBPATCH = 4
 
 logger = logging.getLogger(__name__)
 
@@ -799,7 +799,11 @@ def _from_file(self, root_path: Path, file_path: Path) -> List[dict]:
                         alert_rule["labels"] = {}
 
                     if self.topology:
-                        alert_rule["labels"].update(self.topology.label_matcher_dict)
+                        # only insert labels that do not already exist
+                        for label, val in self.topology.label_matcher_dict.items():
+                            if label not in alert_rule["labels"]:
+                                alert_rule["labels"][label] = val
+
                         # insert juju topology filters into a prometheus alert rule
                         # logql doesn't like empty matchers, so add a job matcher which hits
                         # any string as a "wildcard" which the topology labels will