title: clear prometheus metrics from collector
body: <p>I'm trying to modify prometheus mesos exporter to expose framework states:
  <a href="https://github.com/mesos/mesos_exporter/pull/97/files" rel="nofollow noreferrer">https://github.com/mesos/mesos_exporter/pull/97/files</a></p>
  <p>A bit about mesos exporter - it collects data from both mesos <code>/metrics/snapshot</code> endpoint, and <code>/state</code> endpoint.
  The issue with the latter, both with the changes in my PR and with existing metrics reported on slaves, is that metrics created lasts for ever (until exporter is restarted).
  So if for example a framework was completed, the metrics reported for this framework will be stale (e.g. it will still show the framework is using CPU).</p>
  <p>So I'm trying to figure out how I can clear those stale metrics. If I could just clear the entire <code>mesosStateCollector</code> each time before collect is done it would be awesome.
  There is a <code>delete</code> method for the different p8s vectors (e.g. <a href="https://godoc.org/github.com/prometheus/client_golang/prometheus#GaugeVec.Delete" rel="nofollow noreferrer"><code>GaugeVec</code></a>), but in order to delete a metric, I need to not only the label name, but also the label value for the relevant metric.</p>