[mongo] Include database_instance tag in mongodb.can_connect service check (#23476)

- Both service_check() calls in check() now use _get_service_check_tags()
  instead of self._config.service_check_tags directly, adding the
  database_instance:<resolved_hostname> tag used by the DBM frontend
- Stop clearing _resolved_hostname in _unset_metadata() so the tag
  remains stable across consecutive CRITICAL checks during an outage
- Update tag assertions in test_integration_standalone.py and
  test_integration_shard.py to include the new tag

Co-authored-by: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: jasonmp85

mongo/changelog.d/23476.fixed

@@ -0,0 +1 @@
+Include `database_instance` tag in `mongodb.can_connect` service check.

mongo/datadog_checks/mongo/mongo.py

@@ -296,11 +296,11 @@ def check(self, _):
                 self._schemas.run_job_loop(tags=self._get_tags(include_internal_resource_tags=True))
                 self._query_metrics.run_job_loop(tags=self._get_tags(include_internal_resource_tags=True))
         except CRITICAL_FAILURE as e:
-            self.service_check(SERVICE_CHECK_NAME, AgentCheck.CRITICAL, tags=self._config.service_check_tags)
+            self.service_check(SERVICE_CHECK_NAME, AgentCheck.CRITICAL, tags=self._get_service_check_tags())
             self._unset_metadata()
             raise e  # Let exception bubble up to global handler and show full error in the logs.
         else:
-            self.service_check(SERVICE_CHECK_NAME, AgentCheck.OK, tags=self._config.service_check_tags)
+            self.service_check(SERVICE_CHECK_NAME, AgentCheck.OK, tags=self._get_service_check_tags())
 
     def _refresh_metadata(self):
         if self._mongo_version is None or self._mongo_modules is None:
@@ -323,7 +323,6 @@ def _refresh_metadata(self):
     def _unset_metadata(self):
         self.log.debug('Due to connection failure we will need to reset the metadata.')
         self._mongo_version = None
-        self._resolved_hostname = None
 
     def _collect_metrics(self):
         deployment = self.deployment_type

mongo/tests/test_integration_shard.py

@@ -17,7 +17,12 @@ def test_mongo_arbiter(aggregator, check, instance_arbiter, dd_run_check):
     check = check(instance_arbiter)
     dd_run_check(check)
 
-    tags = [f'host:{common.HOST}', f'port:{common.PORT_ARBITER}', 'db:admin']
+    tags = [
+        f'host:{common.HOST}',
+        f'port:{common.PORT_ARBITER}',
+        'db:admin',
+        f'database_instance:{check._resolved_hostname}',
+    ]
     aggregator.assert_service_check('mongodb.can_connect', status=MongoDb.OK, tags=tags)
 
     metric_names = aggregator.metric_names

mongo/tests/test_integration_standalone.py

@@ -56,7 +56,7 @@ def test_mongo_db_test(aggregator, check, instance_user, dd_run_check):
     check = check(instance_user)
     dd_run_check(check)
 
-    tags = [f'host:{common.HOST}', f'port:{common.PORT1}', 'db:test']
+    tags = [f'host:{common.HOST}', f'port:{common.PORT1}', 'db:test', f'database_instance:{check._resolved_hostname}']
     aggregator.assert_service_check('mongodb.can_connect', status=MongoDb.OK, tags=tags)
 
     metric_names = aggregator.metric_names
@@ -289,5 +289,6 @@ def test_propagate_agent_tags(
                 f'host:{common.HOST}',
                 f'port:{common.PORT1}',
                 'db:test',
+                f'database_instance:{check._resolved_hostname}',
             ] + agent_tags
             aggregator.assert_service_check('mongodb.can_connect', status=MongoDb.OK, tags=expected_tags)