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>

health/health.go

@@ -9,6 +9,7 @@ import (
 	"errors"
 	"fmt"
 	"net/http"
+	"os"
 	"runtime"
 	"sort"
 	"sync"
@@ -18,6 +19,7 @@ import (
 	"tailscale.com/envknob"
 	"tailscale.com/tailcfg"
 	"tailscale.com/types/opt"
+	"tailscale.com/util/cibuild"
 	"tailscale.com/util/mak"
 	"tailscale.com/util/multierr"
 	"tailscale.com/util/set"
@@ -152,6 +154,11 @@ func (t *Tracker) nil() bool {
 	if t != nil {
 		return false
 	}
+	if cibuild.On() {
+		stack := make([]byte, 1<<10)
+		stack = stack[:runtime.Stack(stack, false)]
+		fmt.Fprintf(os.Stderr, "## WARNING: (non-fatal) nil health.Tracker (being strict in CI):\n%s\n", stack)
+	}
 	// TODO(bradfitz): open source our "unexpected" package
 	// and use it here to capture samples of stacks where
 	// t is nil.