Add metrics server

[pddg]

Close #11

See #13 to get more detail.

[ymmt2005]

@pddg I got notified and found that the CI was failing because of CircleCI.
So, I disabled the building on CircleCI and removed the required check for the master branch.
Your next push should result in a successful CI.

[nojima]

このメトリクスの値はリクエスト数と一致するはずなので、requests_total にしたほうが慣習的な気がします

[pddg]

The number of requests and responses do not always seem to match.
This is because if the request is not a correct SOCKS request, these are dropped.

usocksd/socks/server.go

Lines 128 to 156 in 03fb5bd

	var preamble [2]byte
	_, err := io.ReadFull(conn, preamble[:])
	if err != nil {
	fields := well.FieldsFromContext(ctx)
	fields["client_addr"] = conn.RemoteAddr().String()
	fields[log.FnError] = err.Error()
	_ = s.Logger.Error("failed to read preamble", fields)
	return
	}
	connVer := version(preamble[0])
	var destConn net.Conn
	switch connVer {
	case SOCKS4:
	destConn = s.handleSOCKS4(ctx, conn, preamble[1])
	if destConn == nil {
	return
	}
	case SOCKS5:
	destConn = s.handleSOCKS5(ctx, conn, preamble[1])
	if destConn == nil {
	return
	}
	default:
	fields := well.FieldsFromContext(ctx)
	fields["client_addr"] = conn.RemoteAddr().String()
	_ = s.Logger.Error("unknown SOCKS version", fields)
	return
	}

Do you think we should count the number of these incorrect requests?

[nojima]

I recognized that the word "request" is ambiguous.
How about keeping the number of connections as a metric to clarify what counts?

[pddg]

Does that mean that even invalid SOCKS protocol connections count?

[nojima]

Yes

[pddg]

The labels "reason" and "msg" have been removed. These could receive arbitrary error messages and could unnecessarily increase cardinality or contain strings that were not appropriate as labels.

[ymmt2005]

8081?
These days, I use https://github.com/stretchr/testify to write this kind of tests.

[pddg]

I wrote it by copying and pasting, sorry :(
This is a simple struct comparison, and I believe google/go-cmp is more appropriate.

[pddg]

I will fix this, soon.