Prepend "exporter_" to labels that already exist in exported metrics.

If the metrics exported by a process already contain any of a target's
base labels (such as "job" or "instance", but also any manually assigned
target-group label), don't overwrite that label, but instead add a new
label consisting of the original label name prepended with "exporter_".
This is to accomodate intermediate exporter jobs, which might indicate
e.g. the jobs and instances for which they are exporting data.
Merge pull request #277 from prometheus/feature/exporter-prefix

Prepend "exporter_" to labels that already exist in exported metrics.