[DBMON-6256] Fix postgres RDS host resolution (DataDog#22744)

* Fix detecting RDS host resolution

* Add changelog

* Move auto resolution to config builder

* Move to apply_cloud_defaults

Author: eric-weaver

postgres/changelog.d/22744.fixed

@@ -0,0 +1 @@
+Fixed a bug where `cloud_metadata.aws.instance_endpoint` was not populated for auto-detected RDS hosts, which could prevent query metrics from being properly associated with the database instance in the UI

postgres/datadog_checks/postgres/config.py

@@ -29,6 +29,7 @@
 from datadog_checks.base.utils.aws import rds_parse_tags_from_endpoint
 from datadog_checks.base.utils.db.utils import get_agent_host_tags
 from datadog_checks.postgres.features import Feature, FeatureKey, FeatureNames
+from datadog_checks.postgres.util import AWS_RDS_HOSTNAME_SUFFIX
 
 SSL_MODES = {'disable', 'allow', 'prefer', 'require', 'verify-ca', 'verify-full'}
 TABLE_COUNT_LIMIT = 200
@@ -316,6 +317,10 @@ def apply_validated_defaults(args: dict, instance: dict, validation_result: Vali
 
 
 def apply_cloud_defaults(args: dict, instance: dict, validation_result: ValidationResult):
+    # Auto-detect RDS endpoints and backfill instance_endpoint when not explicitly configured
+    if not args['aws'].get('instance_endpoint') and AWS_RDS_HOSTNAME_SUFFIX in args['host']:
+        args['aws']['instance_endpoint'] = args['host']
+
     # AWS backfill and validation
     if (
         not instance.get("aws", {}).get("managed_authentication", None)

postgres/tests/test_config.py

@@ -368,6 +368,40 @@ def test_cloud_validations(mock_check, minimal_instance):
     assert config.azure.managed_authentication.enabled
 
 
+@pytest.mark.parametrize(
+    'rds_host, expected_rds_tag, expected_instance_endpoint',
+    [
+        (
+            'my-cluster.cluster-cfxdfe8cpixl.us-east-1.rds.amazonaws.com',
+            'dbclusteridentifier:my-cluster',
+            'my-cluster.cluster-cfxdfe8cpixl.us-east-1.rds.amazonaws.com',
+        ),
+        (
+            'my-instance.cfxdfe8cpixl.us-east-1.rds.amazonaws.com',
+            'dbinstanceidentifier:my-instance',
+            'my-instance.cfxdfe8cpixl.us-east-1.rds.amazonaws.com',
+        ),
+    ],
+    ids=['cluster_endpoint', 'instance_endpoint'],
+)
+def test_rds_auto_detected_cloud_metadata(
+    mock_check, minimal_instance, rds_host, expected_rds_tag, expected_instance_endpoint
+):
+    """
+    When a user sets host to an RDS endpoint without explicitly setting
+    aws config, build_config should auto-detect the RDS host and populate
+    aws.instance_endpoint so the backend can properly associate query
+    metrics with the RDS resource.
+    """
+    minimal_instance['host'] = rds_host
+    mock_check.instance = minimal_instance
+    mock_check.init_config = {}
+    config, result = build_config(check=mock_check)
+    assert result.valid
+    assert config.aws.instance_endpoint == expected_instance_endpoint
+    assert expected_rds_tag in config.tags
+
+
 def test_relations_validation_fails_if_no_relname_or_regex():
     with pytest.raises(ConfigurationError):
         RelationsManager.validate_relations_config([{"relkind": ["i"]}])