Document executor/merger stats

Also, change the interval to 30s rather than 10s.  There is some
cost to the gear server to calculate the status report, especially
if the queue is long.

Change-Id: Icfe4c6496e45847cdf884f23a06d7186aafdf8e2
diff --git a/doc/source/admin/monitoring.rst b/doc/source/admin/monitoring.rst
index 55f1908..e6e6139 100644
--- a/doc/source/admin/monitoring.rst
+++ b/doc/source/admin/monitoring.rst
@@ -224,6 +224,49 @@
 
       The number of outstanding nodepool requests from Zuul.
 
+.. stat:: zuul.mergers
+
+   Holds metrics related to Zuul mergers.
+
+   .. stat:: online
+      :type: gauge
+
+      The number of Zuul merger processes online.
+
+   .. stat:: jobs_running
+      :type: gauge
+
+      The number of merge jobs running.
+
+   .. stat:: jobs_queued
+      :type: gauge
+
+      The number of merge jobs queued.
+
+.. stat:: zuul.executors
+
+   Holds metrics related to Zuul executors.
+
+   .. stat:: online
+      :type: gauge
+
+      The number of Zuul executor processes online.
+
+   .. stat:: accepting
+      :type: gauge
+
+      The number of Zuul executor processes accepting new jobs.
+
+   .. stat:: jobs_running
+      :type: gauge
+
+      The number of executor jobs running.
+
+   .. stat:: jobs_queued
+      :type: gauge
+
+      The number of executor jobs queued.
+
 
 As an example, given a job named `myjob` in `mytenant` triggered by a
 change to `myproject` on the `master` branch in the `gate` pipeline