patch: runtime: add metrics for timers

[bradfitz]

We're adding metrics to our Go fork temporarily to let us have visibility into pending timers.

per https://groups.google.com/g/golang-dev/c/pN-OyRKLx1c/m/drmJY3gGAAAJ

// TailscaleNumTimers returns the number of timers that are currently
// pending in the runtime across all Ps, as well as the number of "zombie"
// timers.
func TailscaleNumTimers() (total, zombies int) {
	var sum uint32
	var nzombies int32

	// Prevent allp slice changes. This is like retake.
	lock(&allpLock)
	for _, pp := range allp {
		if pp == nil {
			continue
		}
		sum += pp.timers.len.Load()
		nzombies += pp.timers.zombies.Load()
	}
	unlock(&allpLock)

	return int(sum), int(nzombies)
}

[bradfitz]

Also metrics and time.Timer.TailscaleRelease: #189