What did you do?

Added a static config to monitor a webapp exposing a prometheus endpoint

    static_configs:
      - targets: ['someServer:80']
        labels:
          group: 'monitoring'

What did you expect to see?

The target is sitting behind a reverse proxy (haProxy) and we cannot access the target directly.
haProxy expected to see a Host header like this :

Host : someServer

What did you see instead? Under which circumstances?

But instead saw was this :

Host : someServer:80

Although this does not violate the http spec, the spec does allow for default http ports (80 / 443) to be stripped from the Host header. (https://www.w3.org/Protocols/rfc2616/rfc2616-sec14.html#sec14.23).

We needed to modify our haProxy configuration so that it properly matched incoming http requests from prometheus.