Also strip that domain component from the labels collected.

Thanks for the suggestion @jcalderone!

The Blackbox exporter can be used to check whether some services do answer;
We'll use it for our HTTPS endpoints.
Especially handy is it's checking for TLS cert expiry.

We have to explicitely add the port number so the relabel
config can strip it out again.  Else, `instance' seems to
get its value from somewhere else - somewhere our relabel
config does not look at.

We have enough numbers with little value already