Fix start_time_unix_nano for delta collection temporality

Fixes #4009

Author: ocelotl

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

@@ -278,16 +278,16 @@ def collect(
                     is AggregationTemporality.DELTA
                 ):
 
-                    if current_value is None:
-                        return None
-
                     previous_collection_start_nano = (
                         self._previous_collection_start_nano
                     )
                     self._previous_collection_start_nano = (
                         collection_start_nano
                     )
 
+                    if current_value is None:
+                        return None
+
                     return NumberDataPoint(
                         attributes=self._attributes,
                         start_time_unix_nano=previous_collection_start_nano,

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

@@ -16,6 +16,7 @@
 from logging import ERROR
 from platform import system
 from unittest import TestCase
+from time import sleep
 
 from pytest import mark
 
@@ -345,11 +346,43 @@ def test_synchronous_delta_temporality(self):
 
             results.append(reader.get_metrics_data())
 
-        provider.shutdown()
-
         for metrics_data in results:
             self.assertIsNone(metrics_data)
 
+        results = []
+
+        counter.add(1)
+        results.append(reader.get_metrics_data())
+
+        sleep(0.1)
+        results.append(reader.get_metrics_data())
+
+        counter.add(2)
+        results.append(reader.get_metrics_data())
+
+        metric_data_0 = (
+            results[0]
+            .resource_metrics[0]
+            .scope_metrics[0]
+            .metrics[0]
+            .data.data_points[0]
+        )
+        metric_data_2 = (
+            results[2]
+            .resource_metrics[0]
+            .scope_metrics[0]
+            .metrics[0]
+            .data.data_points[0]
+        )
+
+        self.assertIsNone(results[1])
+
+        self.assertGreater(
+            metric_data_2.start_time_unix_nano, metric_data_0.time_unix_nano
+        )
+
+        provider.shutdown()
+
     @mark.skipif(
         system() != "Linux",
         reason=(