document var_metric usage to prevent bugreports that are actually configuration issues

(closes issue #12151)
Reported by: caio1982
Patches:
      DB_metric3.diff uploaded by caio1982 (license 22)


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@106186 65c4cc65-6c06-0410-ace0-fbb531ad65f3