pmdaprometheus: pass PM_ERR_AGAIN for gone-awol metrics
pmdaprometheus: pass PM_ERR_AGAIN for gone-awol metrics

The former PM_ERR_INST is confusing.  PM_ERR_AGAIN is a little better.

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