A prerequisite to #135, this is about the story how to handle scrape query parameter in general.

This should help to implement the blackbox exporter in a cleaner way, cf. https://prometheus.io/docs/instrumenting/writing_exporters/#deployment : “Note that it is only currently possible to write this type of exporter with the Python and Java client libraries (the blackbox exporter which is written in Go is doing the text format by hand, don’t do this).”

It could also help for the mesos exporter use case, see mesos/mesos_exporter#22