util/clientmetrics: add new package to add metrics to the client

And annotate magicsock as a start.

And add localapi and debug handlers with the Prometheus-format
exporter.

Updates #3307

Change-Id: I47c5d535fe54424741df143d052760387248f8d3
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

tstest/integration/tailscaled_deps_test_darwin.go

@@ -30,6 +30,7 @@ import (
 	_ "tailscale.com/types/flagtype"
 	_ "tailscale.com/types/key"
 	_ "tailscale.com/types/logger"
+	_ "tailscale.com/util/clientmetric"
 	_ "tailscale.com/util/multierr"
 	_ "tailscale.com/util/osshare"
 	_ "tailscale.com/version"

tstest/integration/tailscaled_deps_test_freebsd.go

@@ -30,6 +30,7 @@ import (
 	_ "tailscale.com/types/flagtype"
 	_ "tailscale.com/types/key"
 	_ "tailscale.com/types/logger"
+	_ "tailscale.com/util/clientmetric"
 	_ "tailscale.com/util/multierr"
 	_ "tailscale.com/util/osshare"
 	_ "tailscale.com/version"

tstest/integration/tailscaled_deps_test_linux.go

@@ -30,6 +30,7 @@ import (
 	_ "tailscale.com/types/flagtype"
 	_ "tailscale.com/types/key"
 	_ "tailscale.com/types/logger"
+	_ "tailscale.com/util/clientmetric"
 	_ "tailscale.com/util/multierr"
 	_ "tailscale.com/util/osshare"
 	_ "tailscale.com/version"

tstest/integration/tailscaled_deps_test_openbsd.go

@@ -30,6 +30,7 @@ import (
 	_ "tailscale.com/types/flagtype"
 	_ "tailscale.com/types/key"
 	_ "tailscale.com/types/logger"
+	_ "tailscale.com/util/clientmetric"
 	_ "tailscale.com/util/multierr"
 	_ "tailscale.com/util/osshare"
 	_ "tailscale.com/version"

tstest/integration/tailscaled_deps_test_windows.go

@@ -34,6 +34,7 @@ import (
 	_ "tailscale.com/types/flagtype"
 	_ "tailscale.com/types/key"
 	_ "tailscale.com/types/logger"
+	_ "tailscale.com/util/clientmetric"
 	_ "tailscale.com/util/multierr"
 	_ "tailscale.com/util/osshare"
 	_ "tailscale.com/util/winutil"