health: add Tracker type, in prep for removing global variables

This moves most of the health package global variables to a new
`health.Tracker` type.

But then rather than plumbing the Tracker in tsd.System everywhere,
this only goes halfway and makes one new global Tracker
(`health.Global`) that all the existing callers now use.

A future change will eliminate that global.

Updates #11874
Updates #4136

Change-Id: I6ee27e0b2e35f68cb38fecdb3b2dc4c3f2e09d68
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

ipn/ipnlocal/network-lock.go

@@ -59,11 +59,11 @@ type tkaState struct {
 // b.mu must be held.
 func (b *LocalBackend) tkaFilterNetmapLocked(nm *netmap.NetworkMap) {
 	if b.tka == nil && !b.capTailnetLock {
-		health.SetTKAHealth(nil)
+		health.Global.SetTKAHealth(nil)
 		return
 	}
 	if b.tka == nil {
-		health.SetTKAHealth(nil)
+		health.Global.SetTKAHealth(nil)
 		return // TKA not enabled.
 	}
 
@@ -117,9 +117,9 @@ func (b *LocalBackend) tkaFilterNetmapLocked(nm *netmap.NetworkMap) {
 
 	// Check that we ourselves are not locked out, report a health issue if so.
 	if nm.SelfNode.Valid() && b.tka.authority.NodeKeyAuthorized(nm.SelfNode.Key(), nm.SelfNode.KeySignature().AsSlice()) != nil {
-		health.SetTKAHealth(errors.New(healthmsg.LockedOut))
+		health.Global.SetTKAHealth(errors.New(healthmsg.LockedOut))
 	} else {
-		health.SetTKAHealth(nil)
+		health.Global.SetTKAHealth(nil)
 	}
 }
 
@@ -188,7 +188,7 @@ func (b *LocalBackend) tkaSyncIfNeeded(nm *netmap.NetworkMap, prefs ipn.PrefsVie
 				b.logf("Disablement failed, leaving TKA enabled. Error: %v", err)
 			} else {
 				isEnabled = false
-				health.SetTKAHealth(nil)
+				health.Global.SetTKAHealth(nil)
 			}
 		} else {
 			return fmt.Errorf("[bug] unreachable invariant of wantEnabled w/ isEnabled")