net/netmon, net/tsdial: add some link change metrics

Updates #9040

Change-Id: I2c87572d79d2118bcf1f0122eccfe712c1bea9d5
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

cmd/derper/depaware.txt

@@ -136,7 +136,7 @@ tailscale.com/cmd/derper dependencies: (generated by github.com/tailscale/depawa
         tailscale.com/types/structs                                  from tailscale.com/ipn+
         tailscale.com/types/tkatype                                  from tailscale.com/types/key+
         tailscale.com/types/views                                    from tailscale.com/ipn/ipnstate+
-   W    tailscale.com/util/clientmetric                              from tailscale.com/net/tshttpproxy
+        tailscale.com/util/clientmetric                              from tailscale.com/net/tshttpproxy+
         tailscale.com/util/cloudenv                                  from tailscale.com/hostinfo+
    W    tailscale.com/util/cmpver                                    from tailscale.com/net/tshttpproxy
         tailscale.com/util/cmpx                                      from tailscale.com/cmd/derper+

net/netmon/netmon.go

@@ -16,6 +16,7 @@ import (
 
 	"tailscale.com/net/interfaces"
 	"tailscale.com/types/logger"
+	"tailscale.com/util/clientmetric"
 	"tailscale.com/util/set"
 )
 
@@ -369,6 +370,13 @@ func (m *Monitor) debounce() {
 	}
 }
 
+var (
+	metricChangeEq       = clientmetric.NewCounter("netmon_link_change_eq")
+	metricChange         = clientmetric.NewCounter("netmon_link_change")
+	metricChangeTimeJump = clientmetric.NewCounter("netmon_link_change_timejump")
+	metricChangeMajor    = clientmetric.NewCounter("netmon_link_change_major")
+)
+
 // handlePotentialChange considers whether newState is different enough to wake
 // up callers and updates the monitor's state if so.
 //
@@ -380,6 +388,7 @@ func (m *Monitor) handlePotentialChange(newState *interfaces.State, forceCallbac
 	timeJumped := shouldMonitorTimeJump && m.checkWallTimeAdvanceLocked()
 	if !timeJumped && !forceCallbacks && oldState.Equal(newState) {
 		// Exactly equal. Nothing to do.
+		metricChangeEq.Add(1)
 		return
 	}
 
@@ -410,6 +419,13 @@ func (m *Monitor) handlePotentialChange(newState *interfaces.State, forceCallbac
 			delta.Major = true
 		}
 	}
+	metricChange.Add(1)
+	if delta.Major {
+		metricChangeMajor.Add(1)
+	}
+	if delta.TimeJumped {
+		metricChangeTimeJump.Add(1)
+	}
 	for _, cb := range m.cbs {
 		go cb(delta)
 	}

net/tsdial/tsdial.go

@@ -23,6 +23,7 @@ import (
 	"tailscale.com/net/netns"
 	"tailscale.com/types/logger"
 	"tailscale.com/types/netmap"
+	"tailscale.com/util/clientmetric"
 	"tailscale.com/util/mak"
 )
 
@@ -138,16 +139,25 @@ func (d *Dialer) SetNetMon(netMon *netmon.Monitor) {
 	d.netMonUnregister = d.netMon.RegisterChangeCallback(d.linkChanged)
 }
 
+var (
+	metricLinkChangeConnClosed = clientmetric.NewCounter("tsdial_linkchange_closes")
+)
+
 func (d *Dialer) linkChanged(delta *netmon.ChangeDelta) {
 	d.mu.Lock()
 	defer d.mu.Unlock()
+	var anyClosed bool
 	for id, c := range d.activeSysConns {
 		if changeAffectsConn(delta, c) {
+			anyClosed = true
 			d.logf("tsdial: closing system connection %v->%v due to link change", c.LocalAddr(), c.RemoteAddr())
 			go c.Close()
 			delete(d.activeSysConns, id)
 		}
 	}
+	if anyClosed {
+		metricLinkChangeConnClosed.Add(1)
+	}
 }
 
 // changeAffectsConn reports whether the network change delta affects