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.
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