Support calling flush_stats with None context. (#518)

* Support calling flush_stats with None context.

* Fix lint error.

datadog_lambda/metric.py

@@ -125,6 +125,7 @@ def flush_stats(lambda_context=None):
     lambda_stats.flush()
 
     if extension_thread_stats is not None:
+        tags = None
         if lambda_context is not None:
             tags = get_enhanced_metrics_tags(lambda_context)
             split_arn = lambda_context.invoked_function_arn.split(":")

tests/test_metric.py

@@ -7,7 +7,7 @@
 from datadog.api.exceptions import ClientError
 from datetime import datetime, timedelta
 
-from datadog_lambda.metric import lambda_metric
+from datadog_lambda.metric import lambda_metric, flush_stats
 from datadog_lambda.api import decrypt_kms_api_key, KMS_ENCRYPTION_CONTEXT_KEY
 from datadog_lambda.thread_stats_writer import ThreadStatsWriter
 from datadog_lambda.tags import dd_lambda_layer_tag
@@ -101,6 +101,10 @@ def setUp(self):
         self.mock_threadstats_flush_distributions = patcher.start()
         self.addCleanup(patcher.stop)
 
+        patcher = patch("datadog_lambda.metric.extension_thread_stats")
+        self.mock_extension_thread_stats = patcher.start()
+        self.addCleanup(patcher.stop)
+
     def test_retry_on_remote_disconnected(self):
         # Raise the RemoteDisconnected error
         lambda_stats = ThreadStatsWriter(True)
@@ -123,6 +127,10 @@ def test_flush_stats_with_tags(self):
         for tag in tags:
             self.assertTrue(tag in lambda_stats.thread_stats.constant_tags)
 
+    def test_flush_stats_without_context(self):
+        flush_stats(lambda_context=None)
+        self.mock_extension_thread_stats.flush.assert_called_with(None)
+
 
 MOCK_FUNCTION_NAME = "myFunction"