pmdaprometheus: fix exception in store callback
pmdaprometheus: fix exception in store callback

coding error was throwing an exception in the store callback.

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