Stop Heartbeat monitor jobs on cancelation (elastic#20570)

If a monitor is stopped, for example when using autodiscover, the
scheduled tasks should be stopped too. Scheduler was rescheduling tasks
forever once started, though these tasks were not being executed because
they are also aware of the context.

This change avoids the execution and rescheduling of tasks once its job
context is done.

CHANGELOG.next.asciidoc

@@ -247,6 +247,7 @@ https://github.com/elastic/beats/compare/v7.0.0-alpha2...master[Check the HEAD d
 - Fixed excessive memory usage introduced in 7.5 due to over-allocating memory for HTTP checks. {pull}15639[15639]
 - Fixed TCP TLS checks to properly validate hostnames, this broke in 7.x and only worked for IP SANs. {pull}17549[17549]
 - Add support for new `service_name` option to all monitors. {pull}19932[19932].
+- Stop rescheduling tasks of stopped monitors. {pull}20570[20570]
 
 *Journalbeat*
 

heartbeat/scheduler/scheduler.go

@@ -188,6 +188,12 @@ func (s *Scheduler) Add(sched Schedule, id string, entrypoint TaskFunc) (removeF
 	var taskFn timerqueue.TimerTaskFn
 
 	taskFn = func(_ time.Time) {
+		select {
+		case <-jobCtx.Done():
+			debugf("Job '%v' canceled", id)
+			return
+		default:
+		}
 		s.stats.activeJobs.Inc()
 		lastRanAt = s.runRecursiveJob(jobCtx, entrypoint)
 		s.stats.activeJobs.Dec()