Build metrics-reporter-prometheus plugin on stable-2.15

Change-Id: I6c717e8201f8fd80ccf7e785f915406678f0bc44
diff --git a/jenkins/gerrit-bazel-plugin-metrics-reporter-prometheus.yaml b/jenkins/gerrit-bazel-plugin-metrics-reporter-prometheus.yaml
index 85256e5..8b2a587 100644
--- a/jenkins/gerrit-bazel-plugin-metrics-reporter-prometheus.yaml
+++ b/jenkins/gerrit-bazel-plugin-metrics-reporter-prometheus.yaml
@@ -4,4 +4,5 @@
       - 'plugin-{name}-bazel-{branch}':
           branch:
             - master
-            - stable-2.14
\ No newline at end of file
+            - stable-2.15
+            - stable-2.14