Add force flush method to metric exporter

Fixes #2850

CHANGELOG.md

@@ -7,6 +7,8 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## [Unreleased]
 
+- Add `force_flush` method to metrics exporter
+  ([#2852](https://github.com/open-telemetry/opentelemetry-python/pull/2852))
 - Change tracing to use `Resource.to_json()`
   ([#2784](https://github.com/open-telemetry/opentelemetry-python/pull/2784))
 - Fix get_log_emitter instrumenting_module_version args typo

exporter/opentelemetry-exporter-otlp-proto-grpc/src/opentelemetry/exporter/otlp/proto/grpc/metric_exporter/__init__.py

@@ -210,3 +210,6 @@ def shutdown(self, timeout_millis: float = 30_000, **kwargs) -> None:
     @property
     def _exporting(self) -> str:
         return "metrics"
+
+    def force_flush(self, timeout_millis: float = 10_000) -> bool:
+        return True

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

@@ -85,6 +85,13 @@ def export(
             The result of the export
         """
 
+    @abstractmethod
+    def force_flush(self, timeout_millis: float = 10_000) -> bool:
+        """
+        Ensure that export of any metrics currently received by the exporter
+        are completed as soon as possible.
+        """
+
     @abstractmethod
     def shutdown(self, timeout_millis: float = 30_000, **kwargs) -> None:
         """Shuts down the exporter.
@@ -125,6 +132,9 @@ def export(
     def shutdown(self, timeout_millis: float = 30_000, **kwargs) -> None:
         pass
 
+    def force_flush(self, timeout_millis: float = 10_000) -> bool:
+        return True
+
 
 class MetricReader(ABC):
     """