Add silenced_at field to alert group public API (#3992)

# What this PR does

## Which issue(s) this PR fixes

Closes #3913 

## Checklist

- [x] Unit, integration, and e2e (if applicable) tests updated
- [x] Documentation added (or `pr:no public docs` PR label added if not
required)
- [x] `CHANGELOG.md` updated (or `pr:no changelog` PR label added if not
required)

Co-authored-by: Joey Orlando <joey.orlando@grafana.com>

docs/sources/oncall-api-reference/alertgroups.md

@@ -36,7 +36,8 @@ The above command returns JSON structured in the following way:
       "permalinks": {
         "slack": "https://ghostbusters.slack.com/archives/C1H9RESGA/p135854651500008",
         "telegram": "https://t.me/c/5354/1234?thread=1234"
-      }
+      },
+      "silenced_at": "2020-05-19T13:37:01.429805Z",
     }
   ],
   "current_page_number": 1,

engine/apps/public_api/serializers/incidents.py

@@ -43,6 +43,7 @@ class IncidentSerializer(EagerLoadingMixin, serializers.ModelSerializer):
             "acknowledged_by",
             "title",
             "permalinks",
+            "silenced_at",
         ]
 
     def get_title(self, obj):

engine/apps/public_api/tests/test_alert_groups.py

@@ -29,6 +29,11 @@ def construct_expected_response_from_alert_groups(alert_groups):
             acknowledged_at = alert_group.acknowledged_at.isoformat()
             acknowledged_at = acknowledged_at[:-6] + "Z"
 
+        silenced_at = None
+        if alert_group.silenced_at:
+            silenced_at = alert_group.silenced_at.isoformat()
+            silenced_at = silenced_at[:-6] + "Z"
+
         def user_pk_or_none(alert_group, user_field):
             u = getattr(alert_group, user_field)
             if u is not None:
@@ -52,6 +57,7 @@ def construct_expected_response_from_alert_groups(alert_groups):
                     "telegram": None,
                     "web": alert_group.web_link,
                 },
+                "silenced_at": silenced_at,
             }
         )
     return {