Fix EMF exporter to generate EMF metrics with dimension empty (#434)

*Description of changes:*
* Populate the correct `CloudWatchMetrics` in EMF when metrics has empty
dimension list

*Test*
```
{
    "_aws": {
        "Timestamp": 1753139679038,
        "CloudWatchMetrics": [
            {
                "Namespace": "MyApplication1",
                "Metrics": [
                    {
                        "Name": "system_cpu_usage_percent",
                        "Unit": "Percent"
                    },
                    {
                        "Name": "http_request_duration_seconds",
                        "Unit": "Seconds"
                    }
                ]
            }
        ]
    },
    "Version": "1",
    "otel.resource.telemetry.sdk.language": "python",
    "otel.resource.telemetry.sdk.name": "opentelemetry",
    "otel.resource.telemetry.sdk.version": "1.33.1",
    "otel.resource.service.name": "my-service",
    "otel.resource.service.version": "0.1.0",
    "otel.resource.deployment.environment": "production",
    "system_cpu_usage_percent": 0.2,
    "http_request_duration_seconds": {
        "Values": [
            0.39614037455626866,
            0.4866507397084614
        ],
        "Counts": [
            1,
            1
        ],
        "Count": 2,
        "Sum": 0.8848342305738114,
        "Max": 0.4877384525234254,
        "Min": 0.397095778050386
    }
}
```


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Author: mxiamxia

aws-opentelemetry-distro/src/amazon/opentelemetry/distro/exporter/aws/metrics/aws_cloudwatch_emf_exporter.py

@@ -509,11 +509,13 @@ def _create_emf_log(
         for name, value in all_attributes.items():
             emf_log[name] = str(value)
 
-        # Add the single dimension set to CloudWatch Metrics if we have dimensions and metrics
-        if dimension_names and metric_definitions:
-            emf_log["_aws"]["CloudWatchMetrics"].append(
-                {"Namespace": self.namespace, "Dimensions": [dimension_names], "Metrics": metric_definitions}
-            )
+        # Add CloudWatch Metrics if we have metrics, include dimensions only if they exist
+        if metric_definitions:
+            cloudwatch_metric = {"Namespace": self.namespace, "Metrics": metric_definitions}
+            if dimension_names:
+                cloudwatch_metric["Dimensions"] = [dimension_names]
+
+            emf_log["_aws"]["CloudWatchMetrics"].append(cloudwatch_metric)
 
         return emf_log
 

aws-opentelemetry-distro/tests/amazon/opentelemetry/distro/exporter/aws/metrics/test_aws_cloudwatch_emf_exporter.py

@@ -391,9 +391,38 @@ def test_create_emf_log_with_resource(self):
         # Check CloudWatch metrics structure
         cw_metrics = result["_aws"]["CloudWatchMetrics"][0]
         self.assertEqual(cw_metrics["Namespace"], "TestNamespace")
+        self.assertIn("Dimensions", cw_metrics)
         self.assertEqual(set(cw_metrics["Dimensions"][0]), {"env", "service"})
         self.assertEqual(cw_metrics["Metrics"][0]["Name"], "gauge_metric")
 
+    def test_create_emf_log_without_dimensions(self):
+        """Test EMF log creation with metrics but no dimensions."""
+        # Create test record without attributes (no dimensions)
+        gauge_record = self.exporter._create_metric_record("gauge_metric", "Count", "Gauge")
+        gauge_record.value = 75.0
+        gauge_record.timestamp = int(time.time() * 1000)
+        gauge_record.attributes = {}  # No attributes = no dimensions
+
+        records = [gauge_record]
+        resource = Resource.create({"service.name": "test-service"})
+
+        result = self.exporter._create_emf_log(records, resource, 1234567890)
+
+        # Verify EMF log structure
+        self.assertIn("_aws", result)
+        self.assertIn("CloudWatchMetrics", result["_aws"])
+        self.assertEqual(result["_aws"]["Timestamp"], 1234567890)
+        self.assertEqual(result["Version"], "1")
+
+        # Check metric value
+        self.assertEqual(result["gauge_metric"], 75.0)
+
+        # Check CloudWatch metrics structure - should have metrics but no dimensions
+        cw_metrics = result["_aws"]["CloudWatchMetrics"][0]
+        self.assertEqual(cw_metrics["Namespace"], "TestNamespace")
+        self.assertNotIn("Dimensions", cw_metrics)  # No dimensions should be present
+        self.assertEqual(cw_metrics["Metrics"][0]["Name"], "gauge_metric")
+
     def test_create_emf_log_skips_empty_metric_names(self):
         """Test that EMF log creation skips records with empty metric names."""
         # Create a record with no metric name