pmdaprometheus: use c_api for label constants as well
pmdaprometheus: use c_api for label constants as well

Resolves next little grouping of pylint warnings.

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