pmdaprometheus: suppress instance naming prefixing for pcp passthrough
pmdaprometheus: suppress instance naming prefixing for pcp passthrough

Metrics that are incoming from # PCP (pmwebd) sources must not be
subjected to instance-number prefixing.

adfernandes/pcp,adfernandes/pcp,adfernandes/pcp,adfernandes/pcp,adfernandes/pcp,adfernandes/pcp,adfernandes/pcp,adfernandes/pcp