Allow empty metric descriptor label keys (#611)

Author: samn

opencensus/metrics/export/metric_descriptor.py

@@ -130,8 +130,8 @@ def __init__(self, name, description, unit, type_, label_keys):
         if type_ not in MetricDescriptorType:
             raise ValueError("Invalid type")
 
-        if not label_keys:
-            raise ValueError("label_keys must not be empty or null")
+        if label_keys is None:
+            raise ValueError("label_keys must not be None")
 
         if any(key is None for key in label_keys):
             raise ValueError("label_keys must not contain null keys")

tests/unit/metrics/export/test_metric_descriptor.py

@@ -53,6 +53,11 @@ def test_null_label_keys(self):
                 NAME, DESCRIPTION, UNIT,
                 metric_descriptor.MetricDescriptorType.GAUGE_DOUBLE, None)
 
+    def test_empty_label_keys(self):
+        metric_descriptor.MetricDescriptor(
+            NAME, DESCRIPTION, UNIT,
+            metric_descriptor.MetricDescriptorType.GAUGE_DOUBLE, [])
+
     def test_null_label_key_values(self):
         with self.assertRaises(ValueError):
             metric_descriptor.MetricDescriptor(