ipn/{ipnlocal,localapi},wgengine{,/magicsock}: plumb health.Tracker

Down to 25 health.Global users. After this remains controlclient &
net/dns & wgengine/router.

Updates #11874
Updates #4136

Change-Id: I6dd1856e3d9bf523bdd44b60fb3b8f7501d5dc0d
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

ipn/ipnlocal/local.go

@@ -327,6 +327,16 @@ type LocalBackend struct {
 	outgoingFiles map[string]*ipn.OutgoingFile
 }
 
+// HealthTracker returns the health tracker for the backend.
+func (b *LocalBackend) HealthTracker() *health.Tracker {
+	return b.health
+}
+
+// NetMon returns the network monitor for the backend.
+func (b *LocalBackend) NetMon() *netmon.Monitor {
+	return b.sys.NetMon.Get()
+}
+
 type updateStatus struct {
 	started bool
 }