Move counter_source definition

The source of a Counter is not tight to the collector publishing system at
all, so move that a level above, otherwise code which don't import the
collector will fail using this variable.

Change-Id: Ie70b9b544505a8c7d9841163a7d5960cf677f3e4
Signed-off-by: Julien Danjou <julien@danjou.info>
Merge "Move counter_source definition"