Update libmetrics docs to cover some recent questions and issues.

README

 Copyright (c) 2010 The Chromium OS Authors. All rights reserved.
 Use of this source code is governed by a BSD-style license that can be
 found in the LICENSE file.
 
 The Chrome OS "metrics" package contains utilities for client-side
 user metric collection. The collected data is sent to Chrome for
 transport to the UMA server.
 
 
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 screen saver states to determine if the user is actively using the
 device or not and generates the corresponding data. The metrics daemon
 uses libmetrics to send the data to Chrome.
 
 The recommended way to generate metrics data from a module is to link
 and use libmetrics directly. However, the module could instead send
 signals to or communicate in some alternative way with the metrics
 daemon. Then the metrics daemon needs to monitor for the relevant
 events and take appropriate action -- for example, aggregate data and
 send the histogram samples.
+
+
+================================================================================
+FAQ
+================================================================================
+
+Q. What should my histogram's |min| and |max| values be set at?
+
+A. You should set the values to a range that covers the vast majority
+   of samples that would appear in the field. Note that samples below
+   the |min| will still be collected in the underflow bucket and
+   samples above the |max| will end up in the overflow bucket. Also,
+   the reported mean of the data will be correct regardless of the
+   range.
+
+Q. How many buckets should I use in my histogram?
+
+A. You should allocate as many buckets as necessary to perform proper
+   analysis on the collected data. Note, however, that the memory
+   allocated in Chrome for each histogram is proportional to the
+   number of buckets. Therefore, it is strongly recommended to keep
+   this number low (e.g., 50 is normal, while 100 is probably high).
+
+Q. When should I use an enumeration (linear) histogram vs. a regular
+   (exponential) histogram?
+
+A. Enumeration histograms should really be used only for sampling
+   enumerated events and, in some cases, percentages. Normally, you
+   should use a regular histogram with exponential bucket layout that
+   provides higher resolution at the low end of the range and lower
+   resolution at the high end. Regular histograms are generally used
+   for collecting performance data (e.g., timing, memory usage, power)
+   as well as aggregated event counts.