Add possiblity to set CLUSTER_ID via env var

- mitigate missing alert_relabel_configs in Cortex
- if missing in the alert labels it will be added

Change-Id: I89bcc0672aa663178d02e69bdc941c1fcc63db80
Related-PROD: PRODX-24796
diff --git a/sf_notifier/helpers.py b/sf_notifier/helpers.py
index 6965b70..4dc4048 100644
--- a/sf_notifier/helpers.py
+++ b/sf_notifier/helpers.py
@@ -53,9 +53,13 @@
     return '{}\n\n{}:\n{}'.format(description_old, msg, prometheus_url)
 
 
-def alert_fields_and_action(alert, add_links=False):
+def alert_fields_and_action(alert, add_links=False, cluster_id=None):
     fields = []
 
+    if cluster_id is not None:
+        if alert['labels'].get('cluster_id') is None:
+            alert['labels']['cluster_id'] = cluster_id
+
     if alert['status'] in RESOLVED_STATUSES:
         action = 'close_case'
         fields.append(alert['labels'])