Exporter/Prometheus: Convert None label values to empty string. (census-instrumentation#486)

* Exporter/Prometheus: Convert None label values to empty string.

* Use list comprehension.

Author: songy23

opencensus/stats/exporters/prometheus_exporter.py

@@ -166,11 +166,16 @@ def to_metric(self, desc, tag_values, agg_data):
         metric_description = desc['documentation']
         label_keys = desc['labels']
 
+        # Prometheus requires that all tag values be strings hence
+        # the need to cast none to the empty string before exporting. See
+        # https://github.com/census-instrumentation/opencensus-python/issues/480
+        tag_values = [tv if tv else "" for tv in tag_values]
+
         if isinstance(agg_data, aggregation_data_module.CountAggregationData):
             metric = CounterMetricFamily(name=metric_name,
                                          documentation=metric_description,
                                          labels=label_keys)
-            metric.add_metric(labels=list(tag_values),
+            metric.add_metric(labels=tag_values,
                               value=agg_data.count_data)
             return metric
 
@@ -186,7 +191,7 @@ def to_metric(self, desc, tag_values, agg_data):
             metric = HistogramMetricFamily(name=metric_name,
                                            documentation=metric_description,
                                            labels=label_keys)
-            metric.add_metric(labels=list(tag_values),
+            metric.add_metric(labels=tag_values,
                               buckets=list(points.items()),
                               sum_value=agg_data.sum,)
             return metric
@@ -196,7 +201,7 @@ def to_metric(self, desc, tag_values, agg_data):
             metric = UntypedMetricFamily(name=metric_name,
                                          documentation=metric_description,
                                          labels=label_keys)
-            metric.add_metric(labels=list(tag_values),
+            metric.add_metric(labels=tag_values,
                               value=agg_data.sum_data)
             return metric
 
@@ -205,7 +210,7 @@ def to_metric(self, desc, tag_values, agg_data):
             metric = GaugeMetricFamily(name=metric_name,
                                        documentation=metric_description,
                                        labels=label_keys)
-            metric.add_metric(labels=list(tag_values),
+            metric.add_metric(labels=tag_values,
                               value=agg_data.value)
             return metric
 

tests/unit/stats/exporter/test_prometheus_stats.py

@@ -240,6 +240,25 @@ def test_collector_collect(self):
         self.assertEqual('histogram', metric.type)
         self.assertEqual(4, len(metric.samples))
 
+    def test_collector_collect_with_none_label_value(self):
+        agg = aggregation_module.LastValueAggregation(256)
+        view = view_module.View("new_view", "processed video size over time",
+                                [FRONTEND_KEY], VIDEO_SIZE_MEASURE, agg)
+        registry = mock.Mock()
+        options = prometheus.Options("test3", 8001, "localhost", registry)
+        collector = prometheus.Collector(options=options)
+        collector.register_view(view)
+        desc = collector.registered_views['test3_new_view']
+        metric = collector.to_metric(
+            desc=desc, tag_values=[None], agg_data=agg.aggregation_data)
+
+        self.assertEqual(1, len(metric.samples))
+        sample = metric.samples[0]
+        # Sample is a namedtuple
+        # ('Sample', ['name', 'labels', 'value', 'timestamp', 'exemplar'])
+        label_map = sample[1]
+        self.assertEqual({"myorg_keys_frontend": ""}, label_map)
+
 
 class TestPrometheusStatsExporter(unittest.TestCase):
     def test_exporter_constructor_no_namespace(self):