routePrefixed override

[kwilt]

This new parameter will allow for serving the landing page when using --web.external-url as an argument with many exporters.

I added this as it is needed for this pull request: prometheus/snmp_exporter#1335

So in practice it will look something like this:

landingConfig := web.LandingConfig{
        // config stuff goes here ...
		}
                                                                      // new parameter
		landingPage, err := web.NewLandingPage(landingConfig, *routePrefix != "")
		if err != nil {
			logger.Error("Error creating landing page", "err", err)
			os.Exit(1)
		}
		http.Handle(*routePrefix, landingPage)

[SuperQ]

We already have a LandingConfig, it doesn't make sense to add another bool value here.

Also, rather than have a bool value, we can add a RoutePrefix string directly to the LandingConfig. If the RoutePrefix is "", we can default it to /.

[kwilt]

Okay, I think I've made the changes you requested