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_db_default.go

@@ -0,0 +1,26 @@
+// Copyright (c) Tailscale Inc & AUTHORS
+// SPDX-License-Identifier: BSD-3-Clause
+
+//go:build !(windows && 386)
+
+package main
+
+import (
+	"database/sql"
+
+	_ "modernc.org/sqlite"
+)
+
+type db struct {
+	*sql.DB
+}
+
+func newDB(path string) (*db, error) {
+	d, err := sql.Open("sqlite", *flagOut)
+	if err != nil {
+		return nil, err
+	}
+	return &db{
+		DB: d,
+	}, nil
+}