fix(metrics): ensure dimension_set is reused across instances (pointer) (#1581)

Author: heitorlessa

Diff for: aws_lambda_powertools/metrics/metrics.py

@@ -77,7 +77,8 @@ def __init__(self, service: Optional[str] = None, namespace: Optional[str] = Non
         self.namespace: Optional[str] = namespace
         self.metadata_set = self._metadata
         self.default_dimensions = self._default_dimensions
-        self.dimension_set = {**self._default_dimensions, **self._dimensions}
+        self.dimension_set = self._dimensions
+        self.dimension_set.update(**self._default_dimensions)
 
         super().__init__(
             metric_set=self.metric_set,

Diff for: tests/functional/test_metrics.py

@@ -898,3 +898,30 @@ def lambda_handler(evt, ctx):
     # THEN we should have default dimensions in both outputs
     assert "environment" in first_invocation
     assert "environment" in second_invocation
+
+
+def test_metrics_reuse_dimension_set(metric, dimension, namespace):
+    # GIVEN Metrics is initialized with a metric and dimension
+    my_metrics = Metrics(namespace=namespace)
+    my_metrics.add_dimension(**dimension)
+    my_metrics.add_metric(**metric)
+
+    # WHEN Metrics is initialized one more time
+    my_metrics_2 = Metrics(namespace=namespace)
+
+    # THEN both class instances should have the same dimension set
+    assert my_metrics_2.dimension_set == my_metrics.dimension_set
+
+
+def test_metrics_reuse_metadata_set(metric, dimension, namespace):
+    # GIVEN Metrics is initialized with a metric, dimension, and metadata
+    my_metrics = Metrics(namespace=namespace)
+    my_metrics.add_dimension(**dimension)
+    my_metrics.add_metric(**metric)
+    my_metrics.add_metadata(key="meta", value="data")
+
+    # WHEN Metrics is initialized one more time
+    my_metrics_2 = Metrics(namespace=namespace)
+
+    # THEN both class instances should have the same metadata set
+    assert my_metrics_2.metadata_set == my_metrics.metadata_set