derp/derpserver: clean up extraction of derp.Server (#17264)

PR #17258 extracted `derp.Server` into `derp/derpserver.Server`.

This followup patch adds the following cleanups:
1. Rename `derp_server*.go` files to `derpserver*.go` to match
   the package name.
2. Rename the `derpserver.NewServer` constructor to `derpserver.New`
   to reduce stuttering.
3. Remove the unnecessary `derpserver.Conn` type alias.

Updates #17257
Updates #cleanup

Signed-off-by: Simon Law <sfllaw@tailscale.com>

derp/derpserver/derpserver_linux.go

@@ -0,0 +1,91 @@
+// Copyright (c) Tailscale Inc & AUTHORS
+// SPDX-License-Identifier: BSD-3-Clause
+
+//go:build linux && !android
+
+package derpserver
+
+import (
+	"context"
+	"crypto/tls"
+	"net"
+	"time"
+
+	"tailscale.com/net/tcpinfo"
+)
+
+func (c *sclient) startStatsLoop(ctx context.Context) {
+	// Get the RTT initially to verify it's supported.
+	conn := c.tcpConn()
+	if conn == nil {
+		c.s.tcpRtt.Add("non-tcp", 1)
+		return
+	}
+	if _, err := tcpinfo.RTT(conn); err != nil {
+		c.logf("error fetching initial RTT: %v", err)
+		c.s.tcpRtt.Add("error", 1)
+		return
+	}
+
+	const statsInterval = 10 * time.Second
+
+	// Don't launch a goroutine; use a timer instead.
+	var gatherStats func()
+	gatherStats = func() {
+		// Do nothing if the context is finished.
+		if ctx.Err() != nil {
+			return
+		}
+
+		// Reschedule ourselves when this stats gathering is finished.
+		defer c.s.clock.AfterFunc(statsInterval, gatherStats)
+
+		// Gather TCP RTT information.
+		rtt, err := tcpinfo.RTT(conn)
+		if err == nil {
+			c.s.tcpRtt.Add(durationToLabel(rtt), 1)
+		}
+
+		// TODO(andrew): more metrics?
+	}
+
+	// Kick off the initial timer.
+	c.s.clock.AfterFunc(statsInterval, gatherStats)
+}
+
+// tcpConn attempts to get the underlying *net.TCPConn from this client's
+// Conn; if it cannot, then it will return nil.
+func (c *sclient) tcpConn() *net.TCPConn {
+	nc := c.nc
+	for {
+		switch v := nc.(type) {
+		case *net.TCPConn:
+			return v
+		case *tls.Conn:
+			nc = v.NetConn()
+		default:
+			return nil
+		}
+	}
+}
+
+func durationToLabel(dur time.Duration) string {
+	switch {
+	case dur <= 10*time.Millisecond:
+		return "10ms"
+	case dur <= 20*time.Millisecond:
+		return "20ms"
+	case dur <= 50*time.Millisecond:
+		return "50ms"
+	case dur <= 100*time.Millisecond:
+		return "100ms"
+	case dur <= 150*time.Millisecond:
+		return "150ms"
+	case dur <= 250*time.Millisecond:
+		return "250ms"
+	case dur <= 500*time.Millisecond:
+		return "500ms"
+	default:
+		return "inf"
+	}
+}