Verify previous point is returned for cumulative instruments

Fixes open-telemetry#2755

exporter/opentelemetry-exporter-prometheus/src/opentelemetry/exporter/prometheus/__init__.py

@@ -76,6 +76,9 @@
     HistogramMetricFamily,
 )
 from prometheus_client.core import Metric as PrometheusMetric
+from opentelemetry.sdk.metrics._internal.aggregation import (
+    AggregationTemporality
+)
 
 from opentelemetry.sdk.metrics.export import (
     Gauge,
@@ -85,6 +88,7 @@
     MetricsData,
     Sum,
 )
+from opentelemetry.sdk.metrics.view import Aggregation
 
 _logger = getLogger(__name__)
 
@@ -112,8 +116,18 @@ class PrometheusMetricReader(MetricReader):
             the metric belongs to.
     """
 
-    def __init__(self, prefix: str = "") -> None:
-        super().__init__()
+    def __init__(
+        self,
+        preferred_temporality: Dict[type, AggregationTemporality] = None,
+        preferred_aggregation: Dict[
+            type, Aggregation
+        ] = None,
+        prefix: str = ""
+    ) -> None:
+        super().__init__(
+            preferred_temporality=preferred_temporality,
+            preferred_aggregation=preferred_aggregation
+        )
         self._collector = _CustomCollector(prefix)
         REGISTRY.register(self._collector)
         self._collector._callback = self.collect

exporter/opentelemetry-exporter-prometheus/tests/test_prometheus_exporter.py

@@ -18,6 +18,7 @@
 
 from prometheus_client import generate_latest
 from prometheus_client.core import CounterMetricFamily, GaugeMetricFamily
+from opentelemetry.sdk.metrics import MeterProvider
 
 from opentelemetry.exporter.prometheus import (
     PrometheusMetricReader,
@@ -37,6 +38,7 @@
     _generate_sum,
     _generate_unsupported_metric,
 )
+from opentelemetry.sdk.metrics import Counter
 
 
 class TestPrometheusMetricReader(TestCase):
@@ -51,7 +53,7 @@ def setUp(self):
     def test_constructor(self):
         """Test the constructor."""
         with self._registry_register_patch:
-            exporter = PrometheusMetricReader("testprefix")
+            exporter = PrometheusMetricReader(prefix="testprefix")
             self.assertEqual(exporter._collector._prefix, "testprefix")
             self.assertTrue(self._mock_registry_register.called)
 
@@ -286,3 +288,33 @@ def test_check_value(self):
         self.assertEqual(collector._check_value(True), "true")
         self.assertEqual(collector._check_value(False), "false")
         self.assertEqual(collector._check_value(None), "null")
+
+    def test_multiple_collection_calls_1(self):
+
+        metric_reader = PrometheusMetricReader(
+            prefix="prefix",
+            preferred_temporality={Counter: AggregationTemporality.CUMULATIVE}
+        )
+        provider = MeterProvider(metric_readers=[metric_reader])
+        meter = provider.get_meter("getting-started", "0.1.2")
+        counter = meter.create_counter("counter")
+        counter.add(1)
+        result_0 = list(metric_reader._collector.collect())
+        result_1 = list(metric_reader._collector.collect())
+        result_2 = list(metric_reader._collector.collect())
+        self.assertEqual(result_0, result_1)
+        self.assertEqual(result_1, result_2)
+
+        metric_reader = PrometheusMetricReader(
+            prefix="prefix",
+            preferred_temporality={Counter: AggregationTemporality.DELTA}
+        )
+        provider = MeterProvider(metric_readers=[metric_reader])
+        meter = provider.get_meter("getting-started", "0.1.2")
+        counter = meter.create_counter("counter")
+        counter.add(1)
+        result_0 = list(metric_reader._collector.collect())
+        result_1 = list(metric_reader._collector.collect())
+        result_2 = list(metric_reader._collector.collect())
+        self.assertNotEqual(result_0, result_1)
+        self.assertEqual(result_1, result_2)