--web.route-prefix is not propagated to UI

[Ghabi-ISC]

I wanted to run promlens under a subpath as it is possible with prometheus. It is not officially documented. But checking the code I was able to figure out the option --web.route-prefix which is supposed to solve this requirement. Goal is simply to run promlens under a URL like: "https://my.host.com/**supath/to/promlens**"

Unfortunately the configuration is set only in the backend (Go Part) and the Frontend (React) doesn't get the configuration delivered. Debugging it showed that the Variable GLOBAL_PATH_PREFIX doesn't get set correctly and it is always empty. The UI tries to load configuration from "https://my.host.com/", a completly different machine, where nothing could be found.