util: add missing doc on latched counter

Problem
The faq section was deleted accidentally when we were
refactoring the doc bird links in an earlier branch.

Solution
Add back the missing faq.

Differential Revision: https://phabricator.twitter.biz/D750596

doc/src/sphinx/util-stats/faq.rst

@@ -52,3 +52,29 @@ illustrates the advantages of `finagle-stats` over the older libraries.
 histograms. This is available on the
 `/admin/histogram <https://twitter.github.io/twitter-server/Admin.html#admin-histograms>`_
 admin page.
+
+Are metric values reset or latched after an observability collection?
+---------------------------------------------------------------------
+``Stats`` are latched by all three implementations.
+This means that for each observability collection window,
+you will see that window’s distribution of values.
+
+There is no notion of latching/resetting a ``Gauge``, as it is
+always a discrete point in time value. This is the case across
+all three implementations, except for CounterishGauges, which 
+creates a gauge that models an unlatched counter.
+
+``Counters`` have different behavior depending on which implementation
+is being used. This in turn has an impact in how you write your
+queries for your dashboards and alerts. Both Commons Metrics :sup:`[1]` and Commons Stats
+are **not** “latched”, which means that their values are monotonically
+increasing (assuming you never call ``incr()`` with a negative value).
+This implies that you use `CQLs rate function <http://go/cql>`_
+if you want to see per minute counts. Ostrich’s ``Counters`` are latched
+when they are collected (think of this as being a “reset”)
+which means that you should not need to use ``rate`` in order
+to see per minute values.
+
+:sup:`[1]` Note that by using ``finagle-stats`` and setting the flag
+``com.twitter.finagle.stats.useCounterDeltas=true`` you can
+get per-minute deltas in Commons Metrics.