fix(metrics): preserve default_tags when metric-specific tag is set in Datadog provider

aws_lambda_powertools/metrics/provider/datadog/datadog.py

@@ -327,7 +327,11 @@ def _serialize_datadog_tags(metric_tags: Dict[str, Any], default_tags: Dict[str,
             >>> serialize_datadog_tags(metric_tags, None)
             ['environment:production', 'service:web']
         """
-        tags = metric_tags or default_tags
+
+        # We need to create a new dictionary by combining default_tags first,
+        # and then metric_tags on top of it. This ensures that the keys from metric_tags take precedence
+        # and replace corresponding keys in default_tags.
+        tags = {**default_tags, **metric_tags}
 
         return [f"{tag_key}:{tag_value}" for tag_key, tag_value in tags.items()]
 

pyproject.toml

@@ -34,6 +34,7 @@ fastjsonschema = { version = "^2.14.5", optional = true }
 pydantic = { version = "^1.8.2", optional = true }
 boto3 = { version = "^1.20.32", optional = true }
 typing-extensions = "^4.6.2"
+datadog-lambda = { version = "^4.77.0", optional = true }
 
 [tool.poetry.dev-dependencies]
 coverage = {extras = ["toml"], version = "^7.2"}

tests/functional/metrics/test_metrics_datadog.py

@@ -240,6 +240,46 @@ def lambda_handler(evt, ctx):
     assert "environment" in second_invocation
 
 
+def test_log_metrics_precedence_metrics_tags_over_default_tags(capsys):
+    # GIVEN DatadogMetrics is initialized and we persist a set of default tags
+    my_metrics = DatadogMetrics(flush_to_log=True)
+    default_tags = {"environment": "test", "log_group": "/lambda/test"}
+
+    # WHEN we use log_metrics with default_tags to serialize
+    # and create metrics with a tag that has the same name as one of the default_tags
+    @my_metrics.log_metrics(default_tags=default_tags)
+    def lambda_handler(evt, ctx):
+        my_metrics.add_metric(name="item_sold", value=1, environment="metric_precedence")
+
+    lambda_handler({}, {})
+    output = json.loads(capsys.readouterr().out.strip())
+
+    # THEN tag defined in add_metric must have preference over default_tags
+    assert "environment:metric_precedence" in output["t"]
+    assert "environment:test" not in output["t"]
+
+
+def test_log_metrics_merge_metrics_tags_and_default_tags(capsys):
+    # GIVEN DatadogMetrics is initialized and we persist a set of default tags
+    my_metrics = DatadogMetrics(flush_to_log=True)
+    default_tags = {"environment": "test", "log_group": "/lambda/test"}
+
+    # WHEN we use log_metrics with default_tags to serialize
+    # and create metrics with a tag that has the same name as one of the default_tags
+    @my_metrics.log_metrics(default_tags=default_tags)
+    def lambda_handler(evt, ctx):
+        my_metrics.add_metric(name="item_sold", value=1, product="powertools")
+
+    lambda_handler({}, {})
+    output = json.loads(capsys.readouterr().out.strip())
+
+    # THEN there should be serialized default_tags and metric tags
+    output["e"] = ""
+    assert output == json.loads(
+        '{"m":"item_sold","v":1,"e":"","t":["environment:test","log_group:/lambda/test", "product:powertools"]}',
+    )
+
+
 def test_clear_default_tags():
     # GIVEN DatadogMetrics is initialized and we persist a set of default tags
     my_metrics = DatadogMetrics()