Remove console exporter metric output if empty

Fixes #3198

Author: ocelotl

opentelemetry-sdk/src/opentelemetry/sdk/metrics/_internal/export/__init__.py

@@ -344,7 +344,7 @@ def _set_collect_callback(
     @abstractmethod
     def _receive_metrics(
         self,
-        metrics_data: "opentelemetry.sdk.metrics.export.MetricsData",
+        metrics_data: Optional[MetricsData],
         timeout_millis: float = 10_000,
         **kwargs,
     ) -> None:
@@ -386,9 +386,9 @@ def __init__(
             preferred_aggregation=preferred_aggregation,
         )
         self._lock = RLock()
-        self._metrics_data: (
+        self._metrics_data: Optional[
             "opentelemetry.sdk.metrics.export.MetricsData"
-        ) = None
+        ] = None
 
     def get_metrics_data(
         self,
@@ -402,7 +402,7 @@ def get_metrics_data(
 
     def _receive_metrics(
         self,
-        metrics_data: "opentelemetry.sdk.metrics.export.MetricsData",
+        metrics_data: Optional[MetricsData],
         timeout_millis: float = 10_000,
         **kwargs,
     ) -> None:
@@ -511,7 +511,7 @@ def _ticker(self) -> None:
 
     def _receive_metrics(
         self,
-        metrics_data: MetricsData,
+        metrics_data: Optional[MetricsData],
         timeout_millis: float = 10_000,
         **kwargs,
     ) -> None:

opentelemetry-sdk/src/opentelemetry/sdk/metrics/_internal/measurement_consumer.py

@@ -17,7 +17,7 @@
 from abc import ABC, abstractmethod
 from threading import Lock
 from time import time_ns
-from typing import Iterable, List, Mapping
+from typing import List, Mapping, Optional
 
 # This kind of import is needed to avoid Sphinx errors.
 import opentelemetry.sdk.metrics
@@ -29,7 +29,7 @@
 from opentelemetry.sdk.metrics._internal.metric_reader_storage import (
     MetricReaderStorage,
 )
-from opentelemetry.sdk.metrics._internal.point import Metric
+from opentelemetry.sdk.metrics._internal.point import MetricsData
 
 
 class MeasurementConsumer(ABC):
@@ -51,7 +51,7 @@ def collect(
         self,
         metric_reader: "opentelemetry.sdk.metrics.MetricReader",
         timeout_millis: float = 10_000,
-    ) -> Iterable[Metric]:
+    ) -> Optional[MetricsData]:
         pass
 
 
@@ -94,7 +94,7 @@ def collect(
         self,
         metric_reader: "opentelemetry.sdk.metrics.MetricReader",
         timeout_millis: float = 10_000,
-    ) -> Iterable[Metric]:
+    ) -> Optional[MetricsData]:
 
         with self._lock:
             metric_reader_storage = self._reader_storages[metric_reader]

opentelemetry-sdk/src/opentelemetry/sdk/metrics/_internal/metric_reader_storage.py

@@ -15,7 +15,7 @@
 from logging import getLogger
 from threading import RLock
 from time import time_ns
-from typing import Dict, List
+from typing import Dict, List, Optional
 
 from opentelemetry.metrics import (
     Asynchronous,
@@ -119,7 +119,7 @@ def consume_measurement(self, measurement: Measurement) -> None:
         ):
             view_instrument_match.consume_measurement(measurement)
 
-    def collect(self) -> MetricsData:
+    def collect(self) -> Optional[MetricsData]:
         # Use a list instead of yielding to prevent a slow reader from holding
         # SDK locks
 
@@ -231,17 +231,19 @@ def collect(self) -> MetricsData:
                         instrument.instrumentation_scope
                     ].metrics.extend(metrics)
 
-        return MetricsData(
-            resource_metrics=[
-                ResourceMetrics(
-                    resource=self._sdk_config.resource,
-                    scope_metrics=list(
-                        instrumentation_scope_scope_metrics.values()
-                    ),
-                    schema_url=self._sdk_config.resource.schema_url,
-                )
-            ]
-        )
+        scope_metrics = list(instrumentation_scope_scope_metrics.values())
+
+        if scope_metrics:
+
+            return MetricsData(
+                resource_metrics=[
+                    ResourceMetrics(
+                        resource=self._sdk_config.resource,
+                        scope_metrics=scope_metrics,
+                        schema_url=self._sdk_config.resource.schema_url,
+                    )
+                ]
+            )
 
     def _handle_view_instrument_match(
         self,

opentelemetry-sdk/tests/metrics/integration_test/test_console_exporter.py

@@ -72,3 +72,19 @@ def test_console_exporter(self):
 
         self.assertEqual(metrics["attributes"], {"a": "b"})
         self.assertEqual(metrics["value"], 1)
+
+    def test_console_exporter_no_export(self):
+
+        output = StringIO()
+        exporter = ConsoleMetricExporter(out=output)
+        reader = PeriodicExportingMetricReader(
+            exporter, export_interval_millis=100
+        )
+        provider = MeterProvider(metric_readers=[reader])
+        provider.shutdown()
+
+        output.seek(0)
+        actual = "".join(output.readlines())
+        expected = ""
+
+        self.assertEqual(actual, expected)