cmd/stunstamp: implement service to measure DERP STUN RTT (#12241)

stunstamp timestamping includes userspace and SO_TIMESTAMPING kernel
timestamping where available. Measurements are written locally to a
sqlite DB, exposed over an HTTP API, and written to prometheus
via remote-write protocol.

Updates tailscale/corp#20344

Signed-off-by: Jordan Whited <jordan@tailscale.com>

cmd/stunstamp/stunstamp_default.go

@@ -0,0 +1,25 @@
+// Copyright (c) Tailscale Inc & AUTHORS
+// SPDX-License-Identifier: BSD-3-Clause
+
+//go:build !linux
+
+package main
+
+import (
+	"errors"
+	"io"
+	"net"
+	"time"
+)
+
+func getConnKernelTimestamp() (io.ReadWriteCloser, error) {
+	return nil, errors.New("unimplemented")
+}
+
+func measureRTTKernel(conn io.ReadWriteCloser, dst *net.UDPAddr, req []byte) (resp []byte, rtt time.Duration, err error) {
+	return nil, 0, errors.New("unimplemented")
+}
+
+func supportsKernelTS() bool {
+	return false
+}