add unix Socket support to MetricsPort
tor recently got support for
MetricsPort in v0.4.5.1-alpha (#40063 (closed)).
According to the man page MetricsPort does not support unix sockets (like SocksPort or ControlSocket).
Using a socket allows for better access control than for example binding it to localhost and only allowing localhost in MetricsPortPolicy because that would allow any local process to access it.