wgengine/magicsock: add more DERP home clientmetrics

Updates tailscale/corp#18095

Signed-off-by: Andrew Dunham <andrew@du.nham.ca>
Change-Id: I423adca2de0730092394bb5fd5796cd35557d352

wgengine/magicsock/derp.go

@@ -135,6 +135,7 @@ func (c *Conn) pickDERPFallback() int {
 		return pickDERPFallbackForTests()
 	}
 
+	metricDERPHomeFallback.Add(1)
 	return ids[rands.IntN(uint64(uintptr(unsafe.Pointer(c))), len(ids))]
 }
 
@@ -173,6 +174,7 @@ func (c *Conn) maybeSetNearestDERP(report *netcheck.Report) (preferredDERP int)
 		myDerp := c.myDerp
 		c.mu.Unlock()
 		if myDerp != 0 {
+			metricDERPHomeNoChangeNoControl.Add(1)
 			return myDerp
 		}
 
@@ -948,6 +950,7 @@ func (c *Conn) cleanStaleDerp() {
 		}
 		if ad.lastWrite.Before(tooOld) {
 			c.closeDerpLocked(i, "idle")
+			metricDERPStaleCleaned.Add(1)
 			dirty = true
 		} else {
 			someNonHomeOpen = true

wgengine/magicsock/magicsock.go

@@ -3028,6 +3028,17 @@ var (
 	// changed from non-zero to a different non-zero.
 	metricDERPHomeChange = clientmetric.NewCounter("derp_home_change")
 
+	// metricDERPHomeNoChangeNoControl is how many times our DERP home
+	// region did not change because we didn't have an active connection to
+	// the control server.
+	metricDERPHomeNoChangeNoControl = clientmetric.NewCounter("derp_home_no_change_no_control")
+
+	// metricDERPHomeFallback is how many times we picked a DERP fallback.
+	metricDERPHomeFallback = clientmetric.NewCounter("derp_home_fallback")
+
+	// metricDERPStaleCleaned is how many times we closed a stale DERP connection.
+	metricDERPStaleCleaned = clientmetric.NewCounter("derp_stale_cleaned")
+
 	// Disco packets received bpf read path
 	//lint:ignore U1000 used on Linux only
 	metricRecvDiscoPacketIPv4 = clientmetric.NewCounter("magicsock_disco_recv_bpf_ipv4")