Add Executor Merger and Ansible execution statsd counters

This adds the following counters:
- zuul.executor.*.phase.setup.<result> (setup task)
- zuul.executor.*.phase.reset.<result> (reset connection task)
- zuul.executor.*.phase.<phase>.<result> (pre/run/post playbooks)
- zuul.executor.*.merger.['SUCCESS','FAILURE'] (merger status)

The data provided by these counters are not very reliable in the sense
that the failures may not be related to the executor itself and is
instead a legitimate issue with the patch or the job it is running.

However, when averaged out, these counters should help us identify if
a particular executor is exhibiting irregular behavior when compared
to regular patterns or other executors.

Change-Id: Ie430f9935dce94f4b90cffee33695e1eb4d1ca7d
diff --git a/doc/source/admin/monitoring.rst b/doc/source/admin/monitoring.rst
index d43fd03..1c17c28 100644
--- a/doc/source/admin/monitoring.rst
+++ b/doc/source/admin/monitoring.rst
@@ -131,6 +131,14 @@
    component of the key will be replaced with the hostname of the
    executor.
 
+   .. stat:: merger.<result>
+      :type: counter
+
+      Incremented to represent the status of a Zuul executor's merger
+      operations. ``<result>`` can be either ``SUCCESS`` or ``FAILURE``.
+      A failed merge operation which would be accounted for as a ``FAILURE``
+      is what ends up being returned by Zuul as a ``MERGER_FAILURE``.
+
    .. stat:: builds
       :type: counter
 
@@ -148,6 +156,27 @@
       The number of builds currently running on this executor.  This
       includes starting builds.
 
+  .. stat:: phase
+
+     Subtree detailing per-phase execution statistics:
+
+     .. stat:: <phase>
+
+        ``<phase>`` represents a phase in the execution of a job.
+        This can be an *internal* phase (such as ``setup`` or ``cleanup``) as
+        well as *job* phases such as ``pre``, ``run`` or ``post``.
+
+        .. stat:: <result>
+           :type: counter
+
+           A counter for each type of result.
+           These results do not, by themselves, determine the status of a build
+           but are indicators of the exit status provided by Ansible for the
+           execution of a particular phase.
+
+           Example of possible counters for each phase are: ``RESULT_NORMAL``,
+           ``RESULT_TIMED_OUT``, ``RESULT_UNREACHABLE``, ``RESULT_ABORTED``.
+
    .. stat:: load_average
       :type: gauge