Build metrics plugins on Gerrit 2.15

Change-Id: I9797edde5f678f5374732937b772209324729048
diff --git a/jenkins/gerrit-buck-bazel-plugin-metrics-reporter-elasticsearch.yaml b/jenkins/gerrit-buck-bazel-plugin-metrics-reporter-elasticsearch.yaml
index 831514f..abd688e 100644
--- a/jenkins/gerrit-buck-bazel-plugin-metrics-reporter-elasticsearch.yaml
+++ b/jenkins/gerrit-buck-bazel-plugin-metrics-reporter-elasticsearch.yaml
@@ -9,4 +9,5 @@
             - master
           gerrit-branch:
             - master
+            - stable-2.15
             - stable-2.14
diff --git a/jenkins/gerrit-buck-bazel-plugin-metrics-reporter-graphite.yaml b/jenkins/gerrit-buck-bazel-plugin-metrics-reporter-graphite.yaml
index d479964..c6e389a 100644
--- a/jenkins/gerrit-buck-bazel-plugin-metrics-reporter-graphite.yaml
+++ b/jenkins/gerrit-buck-bazel-plugin-metrics-reporter-graphite.yaml
@@ -7,4 +7,5 @@
       - 'plugin-{name}-bazel-{branch}':
           branch:
             - master
+            - stable-2.15
             - stable-2.14
diff --git a/jenkins/gerrit-buck-bazel-plugin-metrics-reporter-jmx.yaml b/jenkins/gerrit-buck-bazel-plugin-metrics-reporter-jmx.yaml
index cdbe8c6..05d1495 100644
--- a/jenkins/gerrit-buck-bazel-plugin-metrics-reporter-jmx.yaml
+++ b/jenkins/gerrit-buck-bazel-plugin-metrics-reporter-jmx.yaml
@@ -9,4 +9,5 @@
             - master
           gerrit-branch:
             - master
+            - stable-2.15
             - stable-2.14