Document the rules for units

Incorporate the rules for specifying sample units based on
https://wiki.openstack.org/wiki/Ceilometer/Units

Change-Id: I8ce2275b0c844393b8f03dd6909f29a96c6f1b80
Signed-off-by: Doug Hellmann <doug.hellmann@dreamhost.com>

doc/source/measurements.rst

@@ -34,7 +34,21 @@ Gauge       Discrete items (floating IPs, image uploads) and fluctuating values
 Delta       Changing over time (bandwidth)
 ==========  ==============================================================================
 
-Units should use common abbreviatons:
+Units
+=====
+
+1. Whenever a volume is to be measured, SI approved units and their
+   approved symbols or abbreviations should be used. Information units
+   should be expressed in bits ('b') or bytes ('B').
+2. For a given meter, the units should NEVER, EVER be changed.
+3. When the measurement does not represent a volume, the unit
+   description should always described WHAT is measured (ie: apples,
+   disk, routers, floating IPs, etc.).
+4. When creating a new meter, if another meter exists measuring
+   something similar, the same units and precision should be used.
+5. Samples (aka "meters" or "counters") should always document their
+   units in Ceilometer (API and Documentation) and new sampling code
+   should not be merged without the appropriate documentation.
 
 ============  ========  ==============  =====
 Dimension     Unit      Abbreviations   Note
@@ -44,8 +58,6 @@ Volume        byte                   B
 Time          seconds                s
 ============  ========  ==============  =====
 
-Information units should be expressed in bits ('b') or bytes ('B').
-
 Here are the meter types by components that are currently implemented:
 
 Compute (Nova)