add unix socket support for prometheus

[master-hax]

this PR allows for listening on a Unix domain socket for prometheus metrics by prefixing the path with unix:

also open to alternative prefixes like unix/ (which is what caddy uses)

some of this approach can probably be reused for #112

example usage:

starting endlessh-go

./endlessh-go -alsologtostderr -enable_prometheus -prometheus_host unix:./coolmetricsocket.socket

try connecting to generate some metrics

ssh -v localhost -o Port=2222

get the metrics

curl -v --unix-socket ./coolmetricsocket.socket whateverhostnameyouwant/metrics

[master-hax]

cc @shizunge do you mind taking a look?

[master-hax]

cc @Bananz0 @JeDaYoshi do you mind taking a look?

[JeDaYoshi]

I'm not part of the project, so I can't act on it.

[master-hax]

gotcha! appreciate the fast response.

i see you have contributed here before though - do you have any suggestions or criticisms for this change?

[master-hax]

cc @olofvndrhr do you mind taking a look?

[shizunge]

I will take a look at this at Sept 19 or 20.

[Z0pyrus]

any updates on this?

[master-hax]

any updates on this?

addressed the comments from my side