Build metrics-reporter-prometheus against gerrit stable-3.6

Bug: Issue 15831
Change-Id: Ief27204e5627627d98d2e31150ab3b9c7e74f99f
diff --git a/jenkins/gerrit-bazel-plugin-metrics-reporter-prometheus.yaml b/jenkins/gerrit-bazel-plugin-metrics-reporter-prometheus.yaml
index 210df10..a882e90 100644
--- a/jenkins/gerrit-bazel-plugin-metrics-reporter-prometheus.yaml
+++ b/jenkins/gerrit-bazel-plugin-metrics-reporter-prometheus.yaml
@@ -8,6 +8,7 @@
           branch:
             - master
           gerrit-branch:
+            - stable-3.6
             - stable-3.5
             - stable-3.4
             - stable-3.3