Build metrics-reporter-graphite on its own 2.16, 3.0 and 3.1 branches

Bug: Issue 12671
Change-Id: Iaea0ff86cb257909b00b9419355ead144a1fe62f
diff --git a/jenkins/gerrit-bazel-plugin-metrics-reporter-graphite.yaml b/jenkins/gerrit-bazel-plugin-metrics-reporter-graphite.yaml
index 6b4a988..31f5753 100644
--- a/jenkins/gerrit-bazel-plugin-metrics-reporter-graphite.yaml
+++ b/jenkins/gerrit-bazel-plugin-metrics-reporter-graphite.yaml
@@ -7,6 +7,8 @@
           gerrit-branch:
             - master
             - stable-3.2
+      - 'plugin-{name}-bazel-{branch}':
+          branch:
             - stable-3.1
             - stable-3.0
             - stable-2.16