tsd, ipnlocal, etc: add tsd.System.HealthTracker, start some plumbing

This adds a health.Tracker to tsd.System, accessible via
a new tsd.System.HealthTracker method.

In the future, that new method will return a tsd.System-specific
HealthTracker, so multiple tsnet.Servers in the same process are
isolated. For now, though, it just always returns the temporary
health.Global value. That permits incremental plumbing over a number
of changes. When the second to last health.Global reference is gone,
then the tsd.System.HealthTracker implementation can return a private
Tracker.

The primary plumbing this does is adding it to LocalBackend and its
dozen and change health calls. A few misc other callers are also
plumbed. Subsequent changes will flesh out other parts of the tree
(magicsock, controlclient, etc).

Updates #11874
Updates #4136

Change-Id: Id51e73cfc8a39110425b6dc19d18b3975eac75ce
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

cmd/tailscaled/debug.go

@@ -21,6 +21,7 @@ import (
 	"time"
 
 	"tailscale.com/derp/derphttp"
+	"tailscale.com/health"
 	"tailscale.com/ipn"
 	"tailscale.com/net/interfaces"
 	"tailscale.com/net/netmon"
@@ -157,6 +158,7 @@ func getURL(ctx context.Context, urlStr string) error {
 }
 
 func checkDerp(ctx context.Context, derpRegion string) (err error) {
+	ht := new(health.Tracker)
 	req, err := http.NewRequestWithContext(ctx, "GET", ipn.DefaultControlURL+"/derpmap/default", nil)
 	if err != nil {
 		return fmt.Errorf("create derp map request: %w", err)
@@ -195,6 +197,8 @@ func checkDerp(ctx context.Context, derpRegion string) (err error) {
 
 	c1 := derphttp.NewRegionClient(priv1, log.Printf, nil, getRegion)
 	c2 := derphttp.NewRegionClient(priv2, log.Printf, nil, getRegion)
+	c1.HealthTracker = ht
+	c2.HealthTracker = ht
 	defer func() {
 		if err != nil {
 			c1.Close()