More accurate meter name and unit for host load averages

Fixes bug 1307482

Previously, the SNMP reported load averages were misleadingly metered
as percentage utilizations.

Now, these samples include a meter name and unit that more accurately
reflect their true meaning - i.e. an exponentially-damped moving average
of the number of running or runnable processes.

Change-Id: I5dbc6f7886ed9a451926b502365ec80322f6b458
(cherry picked from commit 5872c6fd12bbd3711f8a9e68adcfa70e8f8d80bc)