fix(metrics): clarify no-metrics user warning (#1935)

Author: rubenfonseca

Diff for: aws_lambda_powertools/metrics/base.py

@@ -391,7 +391,11 @@ def decorate(event, context):
                     self._add_cold_start_metric(context=context)
             finally:
                 if not raise_on_empty_metrics and not self.metric_set:
-                    warnings.warn("No metrics to publish, skipping", stacklevel=2)
+                    warnings.warn(
+                        "No application metrics to publish. The cold-start metric may be published if enabled. "
+                        "If application metrics should never be empty, consider using 'raise_on_empty_metrics'",
+                        stacklevel=2,
+                    )
                 else:
                     metrics = self.serialize_metric_set()
                     self.clear_metrics()

Diff for: tests/functional/test_metrics.py

@@ -728,7 +728,10 @@ def lambda_handler(evt, context):
         warnings.simplefilter("default")
         lambda_handler({}, {})
         assert len(w) == 1
-        assert str(w[-1].message) == "No metrics to publish, skipping"
+        assert str(w[-1].message) == (
+            "No application metrics to publish. The cold-start metric may be published if enabled. "
+            "If application metrics should never be empty, consider using 'raise_on_empty_metrics'"
+        )
 
 
 def test_log_metrics_with_implicit_dimensions_called_twice(capsys, metric, namespace, service):