Handle empty label values for the pushgateway. (#547)

Fixes #546

Signed-off-by: Brian Brazil <brian.brazil@robustperception.io>

prometheus_client/exposition.py

@@ -362,7 +362,10 @@ def _use_gateway(method, gateway, job, registry, grouping_key, timeout, handler)
 
 
 def _escape_grouping_key(k, v):
-    if '/' in v:
+    if v == "" :
+        # Per https://github.com/prometheus/pushgateway/pull/346.
+        return k + "@base64", "="
+    elif '/' in v:
         # Added in Pushgateway 0.9.0.
         return k + "@base64", base64.urlsafe_b64encode(v.encode("utf-8")).decode("utf-8")
     else:

tests/test_exposition.py

@@ -260,6 +260,13 @@ def test_push_with_groupingkey(self):
         self.assertEqual(self.requests[0][0].headers.get('content-type'), CONTENT_TYPE_LATEST)
         self.assertEqual(self.requests[0][1], b'# HELP g help\n# TYPE g gauge\ng 0.0\n')
 
+    def test_push_with_groupingkey_empty_label(self):
+        push_to_gateway(self.address, "my_job", self.registry, {'a': ''})
+        self.assertEqual(self.requests[0][0].command, 'PUT')
+        self.assertEqual(self.requests[0][0].path, '/metrics/job/my_job/a@base64/=')
+        self.assertEqual(self.requests[0][0].headers.get('content-type'), CONTENT_TYPE_LATEST)
+        self.assertEqual(self.requests[0][1], b'# HELP g help\n# TYPE g gauge\ng 0.0\n')
+
     def test_push_with_complex_groupingkey(self):
         push_to_gateway(self.address, "my_job", self.registry, {'a': 9, 'b': 'a/ z'})
         self.assertEqual(self.requests[0][0].command, 'PUT')