Fix legacy alarm matching_metadata broken by recent fix.

The prefix was being computed incorrectly because meter_name is now
being stripped from kwargs. This patch computes the prefix before
kwargs is modified.

Change-Id: I858dd430a4ce4be5cda41b3dd7faa76b8fdf2cfe
Closes-Bug: #1383521
Merge "Fix legacy alarm matching_metadata broken by recent fix."