Add optional timestamp to metric output

[jcpunk]

Fixes #18

[vvolkl]

Hmm, like this it will unfortunately only work with cvmfs versions older than 2.13.2 - with newer versions of cvmfs the script uses the output of cvmfs_talk -i <repo> metrics prometheus unchanged, in order to avoid the overhead of calling cvmfs_talk for each metric individually. So we'd have to update as well https://github.com/cvmfs/cvmfs/blob/devel/cvmfs/talk.cc#L926, cut another cvmfs release and add yet another version handling case to this script. This doesn't sound too appealing, maybe there is some kind of post-processing that we can do instead?

[jcpunk]

I figured out a way to rework the patch to be cleaner. In theory this should work for all forms of the exporter.