pmdaprometheus: tweak fetch and parse time counters
pmdaprometheus: tweak fetch and parse time counters

Calculate the fetch and parse time deltas using local (per-thread) variables.
modified:   src/pmdas/prometheus/pmdaprometheus.python

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