derp: add TCP RTT metric on Linux (#5949)

Periodically poll the TCP RTT metric from all open TCP connections and
update a (bucketed) histogram metric.

Signed-off-by: Andrew Dunham <andrew@du.nham.ca>
Change-Id: I6214902196b05bf7829c9d0ea501ce0e13d984cf

derp/derp_server_default.go

@@ -0,0 +1,14 @@
+// Copyright (c) 2022 Tailscale Inc & AUTHORS All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+//go:build !linux
+// +build !linux
+
+package derp
+
+import "context"
+
+func (c *sclient) statsLoop(ctx context.Context) error {
+	return nil
+}