Skipped metric for non existing paths in gc.
Skipped metric for non existing paths in gc.

Previously, agent gc would increment the "failed" counter if the path
does not exist, but this should not be an issue. This patch skipped such
paths in both "failed" and "succeeded" counters.

Review: https://reviews.apache.org/r/67423

kaysoky/mesos,chhsia0/mesos,kaysoky/mesos,dforsyth/mesos,abudnik/mesos,jpeach/mesos,gsantovena/mesos,kaysoky/mesos,chhsia0/mesos,jpeach/mesos,Gilbert88/mesos,abudnik/mesos,kaysoky/mesos,dforsyth/mesos,jpeach/mesos,dforsyth/mesos,gsantovena/mesos,kaysoky/mesos,andschwa/mesos,reneploetz/mesos,abudnik/mesos,abudnik/mesos,abudnik/mesos,kaysoky/mesos,jpeach/mesos,gsantovena/mesos,reneploetz/mesos,Gilbert88/mesos,andschwa/mesos,dforsyth/mesos,chhsia0/mesos,gsantovena/mesos,Gilbert88/mesos,dforsyth/mesos,jpeach/mesos,abudnik/mesos,andschwa/mesos,reneploetz/mesos,reneploetz/mesos,abudnik/mesos,gsantovena/mesos,andschwa/mesos,Gilbert88/mesos,jpeach/mesos,reneploetz/mesos,kaysoky/mesos,chhsia0/mesos,Gilbert88/mesos,Gilbert88/mesos,andschwa/mesos,gsantovena/mesos,chhsia0/mesos,andschwa/mesos,dforsyth/mesos,reneploetz/mesos,Gilbert88/mesos,jpeach/mesos,chhsia0/mesos,dforsyth/mesos,gsantovena/mesos,reneploetz/mesos,chhsia0/mesos,andschwa/mesos