all: move network monitoring from wgengine/monitor to net/netmon

We're using it in more and more places, and it's not really specific to
our use of Wireguard (and does more just link/interface monitoring).

Also removes the separate interface we had for it in sockstats -- it's
a small enough package (we already pull in all of its dependencies
via other paths) that it's not worth the extra complexity.

Updates #7621
Updates #7850

Signed-off-by: Mihai Parparita <mihai@tailscale.com>

net/sockstats/sockstats.go

@@ -11,7 +11,7 @@ package sockstats
 import (
 	"context"
 
-	"tailscale.com/net/interfaces"
+	"tailscale.com/net/netmon"
 	"tailscale.com/types/logger"
 )
 
@@ -107,17 +107,10 @@ func GetValidation() *ValidationSockStats {
 	return getValidation()
 }
 
-// LinkMonitor is the interface for the parts of wgengine/mointor's Mon that we
-// need, to avoid the dependency.
-type LinkMonitor interface {
-	InterfaceState() *interfaces.State
-	RegisterChangeCallback(interfaces.ChangeFunc) (unregister func())
-}
-
-// SetLinkMonitor configures the sockstats package to monitor the active
+// SetNetMon configures the sockstats package to monitor the active
 // interface, so that per-interface stats can be collected.
-func SetLinkMonitor(lm LinkMonitor) {
-	setLinkMonitor(lm)
+func SetNetMon(lm *netmon.Monitor) {
+	setNetMon(lm)
 }
 
 // DebugInfo returns a string containing debug information about the tracked