openmetrics v2: merge mapping-valued defaults instead of replacing them

datadog_checks_base/changelog.d/24921.fixed

@@ -0,0 +1 @@
+Stop an OpenMetrics V2 instance that configures ``rename_labels`` from silently discarding the renames its check declares in ``get_default_config``. Mapping-valued defaults are now merged with the instance's mapping entry by entry, with the instance's own entries taking precedence, instead of being replaced wholesale.

datadog_checks_base/datadog_checks/base/checks/openmetrics/v2/base.py

@@ -4,6 +4,7 @@
 from __future__ import annotations
 
 from collections import ChainMap
+from collections.abc import Mapping
 from contextlib import contextmanager
 from pathlib import Path
 from typing import TYPE_CHECKING
@@ -18,8 +19,6 @@
 from .scraper import OpenMetricsScraper
 
 if TYPE_CHECKING:
-    from collections.abc import Mapping
-
     from .metrics_mapping import MetricsMapping, _RawMetricsConfig
 
 
@@ -131,10 +130,25 @@ def get_config_with_defaults(self, config):
         Subclasses that override this method must call ``super().get_config_with_defaults(config)``;
         otherwise the YAML mappings declared via ``METRICS_MAP`` (or discovered by convention) are
         silently skipped.
+
+        The instance config takes precedence over the defaults, option by option. Mapping-valued
+        options are the exception: they are merged entry by entry rather than replaced wholesale,
+        since a ``ChainMap`` resolves keys shallowly and an instance that sets such an option at all
+        would otherwise shadow the whole class default -- silently dropping entries the check
+        depends on, such as the ``rename_labels`` renames that keep endpoint labels off Datadog's
+        reserved tag keys. The instance's own entries still win on a per-entry basis.
         """
         defaults = dict(self.get_default_config())
         if file_metrics := self._load_file_based_metrics(config):
             defaults['metrics'] = list(defaults.get('metrics', [])) + file_metrics
+
+        if merged := {
+            option: {**default, **config[option]}
+            for option, default in defaults.items()
+            if isinstance(default, Mapping) and isinstance(config.get(option), Mapping)
+        }:
+            config = {**config, **merged}
+
         return ChainMap(config, defaults)
 
     def get_default_config(self) -> dict:
@@ -143,6 +157,10 @@ def get_default_config(self) -> dict:
         The returned dict can be mutated by the framework before being wrapped
         in a ``ChainMap``. Avoid returning a shared or instance-level object to avoid
         state leakage between check executions.
+
+        Mapping-valued defaults are merged with any mapping the instance configures for the same
+        option, so a user customizing it adds to these defaults instead of replacing them. Defaults
+        of any other type are replaced outright by an instance-level value.
         """
         return {}
 

datadog_checks_base/tests/base/checks/openmetrics/test_v2/test_interface.py

@@ -31,6 +31,96 @@ def get_default_config(self):
     aggregator.assert_all_metrics_covered()
 
 
+def test_default_config_mapping_merged_with_instance(aggregator, dd_run_check, mock_http_response):
+    """
+    A mapping-valued default is merged with the instance's mapping for the same option, entry by
+    entry. The instance config is layered over the defaults in a `ChainMap`, which resolves keys
+    shallowly, so without the merge an instance that sets `rename_labels` at all would shadow the
+    class default wholesale and silently lose renames the check depends on.
+    """
+
+    class Check(OpenMetricsBaseCheckV2):
+        __NAMESPACE__ = 'test'
+
+        def get_default_config(self):
+            return {'metrics': ['.+'], 'rename_labels': {'foo': 'bar'}}
+
+    mock_http_response(
+        """
+        # HELP go_memstats_alloc_bytes Number of bytes allocated and still in use.
+        # TYPE go_memstats_alloc_bytes gauge
+        go_memstats_alloc_bytes{foo="baz",qux="quux"} 6.396288e+06
+        """
+    )
+    check = Check('test', {}, [{'openmetrics_endpoint': 'test', 'rename_labels': {'qux': 'corge'}}])
+    dd_run_check(check)
+
+    # `bar:baz` is the class default's rename, `corge:quux` the instance's own.
+    aggregator.assert_metric(
+        'test.go_memstats_alloc_bytes',
+        6396288,
+        metric_type=aggregator.GAUGE,
+        tags=['endpoint:test', 'bar:baz', 'corge:quux'],
+    )
+
+    aggregator.assert_all_metrics_covered()
+
+
+def test_default_config_mapping_entry_overridden_by_instance(aggregator, dd_run_check, mock_http_response):
+    """
+    Merging mapping-valued defaults must not cost the ability to override one: an instance entry for
+    the same key as a default entry still wins.
+    """
+
+    class Check(OpenMetricsBaseCheckV2):
+        __NAMESPACE__ = 'test'
+
+        def get_default_config(self):
+            return {'metrics': ['.+'], 'rename_labels': {'foo': 'bar'}}
+
+    mock_http_response(
+        """
+        # HELP go_memstats_alloc_bytes Number of bytes allocated and still in use.
+        # TYPE go_memstats_alloc_bytes gauge
+        go_memstats_alloc_bytes{foo="baz"} 6.396288e+06
+        """
+    )
+    check = Check('test', {}, [{'openmetrics_endpoint': 'test', 'rename_labels': {'foo': 'corge'}}])
+    dd_run_check(check)
+
+    aggregator.assert_metric(
+        'test.go_memstats_alloc_bytes', 6396288, metric_type=aggregator.GAUGE, tags=['endpoint:test', 'corge:baz']
+    )
+
+    aggregator.assert_all_metrics_covered()
+
+
+def test_default_config_mapping_not_shared_between_scrapers(aggregator, dd_run_check, mock_http_response):
+    """
+    The merge must not write back into the mapping `get_default_config` returned, or a check with
+    several scraper configs would accumulate every scraper's custom renames into the shared default.
+    """
+    default_renames = {'foo': 'bar'}
+
+    class Check(OpenMetricsBaseCheckV2):
+        __NAMESPACE__ = 'test'
+
+        def get_default_config(self):
+            return {'metrics': ['.+'], 'rename_labels': default_renames}
+
+    mock_http_response(
+        """
+        # HELP go_memstats_alloc_bytes Number of bytes allocated and still in use.
+        # TYPE go_memstats_alloc_bytes gauge
+        go_memstats_alloc_bytes{foo="baz"} 6.396288e+06
+        """
+    )
+    check = Check('test', {}, [{'openmetrics_endpoint': 'test', 'rename_labels': {'qux': 'corge'}}])
+    dd_run_check(check)
+
+    assert default_renames == {'foo': 'bar'}
+
+
 def test_tag_by_endpoint(aggregator, dd_run_check, mock_http_response):
     mock_http_response(
         """