This allows us to use multiple jobs, one for each additional exporter,
and set their `job_name` accordingly.
`job_name` is exported as `job` label on the resulting metrics.
This allows us to quickly get an understanding what metrics of an
exporter are actually available by simply filtering all metrics by
`{job="$jobname"}`
This is handy to quickly see all metrics exported by a node, without
having to mangle with the already existing `instance` label.
`hostname` is essentially a variant of `instance` but without ports.
