Handle empty label values for the pushgateway. (prometheus#553)

Fixes prometheus#552

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

Author: brian-brazil

simpleclient_pushgateway/src/main/java/io/prometheus/client/exporter/PushGateway.java

@@ -286,7 +286,9 @@ void doRequest(CollectorRegistry registry, String job, Map<String, String> group
 
     if (groupingKey != null) {
       for (Map.Entry<String, String> entry: groupingKey.entrySet()) {
-        if (entry.getValue().contains("/")) {
+        if (entry.getValue().isEmpty()) {
+          url += "/" + entry.getKey() + "@base64/=";
+        } else if (entry.getValue().contains("/")) {
           url += "/" + entry.getKey() + "@base64/" + base64url(entry.getValue());
         } else {
           url += "/" + entry.getKey() + "/" + URLEncoder.encode(entry.getValue(), "UTF-8");

simpleclient_pushgateway/src/test/java/io/prometheus/client/exporter/PushGatewayTest.java

@@ -111,6 +111,17 @@ public void testPushWithMultiGroupingKey() throws IOException {
     pg.push(registry, "j", groupingKey);
   }
 
+  @Test
+  public void testPushWithEmptyLabelGroupingKey() throws IOException {
+    mockServerClient.when(
+        request()
+          .withMethod("PUT")
+          .withPath("/metrics/job/j/l/v/l2@base64/=")
+      ).respond(response().withStatusCode(202));
+    groupingKey.put("l2", "");
+    pg.push(registry, "j", groupingKey);
+  }
+
   @Test
   public void testPushWithGroupingKeyWithSlashes() throws IOException {
     mockServerClient.when(
@@ -192,14 +203,12 @@ public void testDeleteWithGroupingKey() throws IOException {
     pg.delete("j", groupingKey);
   }
 
-
-
   @Test
   public void testOldPushWithoutInstance() throws IOException {
     mockServerClient.when(
         request()
           .withMethod("PUT")
-          .withPath("/metrics/job/j/instance/")
+          .withPath("/metrics/job/j/instance@base64/=")
       ).respond(response().withStatusCode(202));
     pg.push(registry, "j", "");
   }