When turning a HostAndPort into a string, the code will omit the port if the HostAndPort refers to a unix domain socket. However, some unix domain sockets are unnamed and we use "anonymous unix socket" instead. This causes the unix domain socket check in HostAndPort to fail since there is no '/'.

One consequence of this is that when a client connects to the server using a unix domain socket, the client metadata will always emit the remote attribute as "anonymous unix socket:27017" (since the remote is expected to be unnamed) even if we are using a unix domain socket associated with a different port. This extra port number is misleading and confusing, so we should update the logic in HostAndPort to handle anonymous unix sockets.