Add support for --web.route-prefix/external-url

[agrrh]

Host operating system:

Not applicable.

blackbox_exporter version: output of blackbox_exporter -version

Current v0.14.0 version is affected:
https://github.com/prometheus/blackbox_exporter/blob/e7e450c/main.go#L283-L286

What is the blackbox.yml module config.

Not applicable.

What is the prometheus.yml scrape config.

Not applicable.

What logging output did you get from adding &debug=true to the probe URL?

Not applicable.

What did you do that produced an error?

I'm running blackbox_exporter under following address:

http://example.org/exporters/blackbox/

What did you expect to see?

I would like index page links lead me to correct URLs, like:

http://example.org/exporters/blackbox/metrics

What did you see instead?

Broken URLs:

http://example.org/metrics