cmd/sniproxy: add port forwarding and prometheus metrics

1. Add TCP port forwarding.
   For example: ./sniproxy -forwards=tcp/22/github.com
   will forward SSH to github.

   % ssh -i ~/.ssh/id_ecdsa.pem -T git@github.com
   Hi GitHubUser! You've successfully authenticated, but GitHub does not
   provide shell access.

   % ssh -i ~/.ssh/id_ecdsa.pem -T git@100.65.x.y
   Hi GitHubUser! You've successfully authenticated, but GitHub does not
   provide shell access.

2. Additionally export clientmetrics as prometheus metrics for local
   scraping over the tailnet: http://sniproxy-hostname:8080/debug/varz

Updates https://github.com/tailscale/tailscale/issues/1748

Signed-off-by: Denton Gentry <dgentry@tailscale.com>

cmd/sniproxy/sniproxy_test.go

@@ -0,0 +1,37 @@
+// Copyright (c) Tailscale Inc & AUTHORS
+// SPDX-License-Identifier: BSD-3-Clause
+
+package main
+
+import (
+	"strings"
+	"testing"
+
+	"github.com/google/go-cmp/cmp"
+)
+
+func TestPortForwardingArguments(t *testing.T) {
+	tests := []struct {
+		in      string
+		wanterr string
+		want    *portForward
+	}{
+		{"", "", nil},
+		{"bad port specifier", "cannot parse", nil},
+		{"tcp/xyz/example.com", "bad forwarding port", nil},
+		{"tcp//example.com", "bad forwarding port", nil},
+		{"tcp/2112/", "bad destination", nil},
+		{"udp/53/example.com", "unsupported forwarding protocol", nil},
+		{"tcp/22/github.com", "", &portForward{Proto: "tcp", Port: 22, Destination: "github.com"}},
+	}
+	for _, tt := range tests {
+		got, goterr := parseForward(tt.in)
+		if tt.wanterr != "" {
+			if !strings.Contains(goterr.Error(), tt.wanterr) {
+				t.Errorf("f(%q).err = %v; want %v", tt.in, goterr, tt.wanterr)
+			}
+		} else if diff := cmp.Diff(got, tt.want); diff != "" {
+			t.Errorf("Parsed forward (-got, +want):\n%s", diff)
+		}
+	}
+}