After discussion with people outside of Ceilometer, it seems that absolute
is a confusing name. For example, what we call currently 'delta' is called
'absolute' in RRD.
Using gauge seems to be clearer.
Change-Id: I1b686e34a1294cbdd35cdb84e3ab9c6c78df04dd
Signed-off-by: Julien Danjou <julien@danjou.info>