worker: run doWork at fixed rate

Currently controller waits for `pollingInterval` after each `doWork` run that takes `W` time long.
Work rate and metrics counter change rate is therefore equal to `1 / (pollingInterval + W)`
which makes it hard to configure alerts based on the counter change rate thresholds due to unknown `W`.

This change runs controller work cycle at fixed rate of `1 / pollingInterval`.

For the existing deployments the side effect would be reduced work cycle and increased API request rates,
e.g. for `W` of 15s and default `pollingInterval` of 30s the work cycle is reduced from 45s to 30s (-33%).

Signed-off-by: Alexander Yastrebov <alexander.yastrebov@zalando.de>

worker.go

@@ -236,6 +236,8 @@ func startPolling(
 	pollingInterval time.Duration,
 	globalWAFACL string,
 ) {
+	ticker := time.NewTicker(pollingInterval)
+	defer ticker.Stop()
 	for {
 		if errs := doWork(certsProvider, certsPerALB, certTTL, awsAdapter, kubeAdapter, globalWAFACL).Errors(); len(errs) > 0 {
 			for _, err := range errs {
@@ -246,9 +248,9 @@ func startPolling(
 		}
 		firstRun = false
 
-		log.Debugf("Start polling sleep %s", pollingInterval)
 		select {
-		case <-time.After(pollingInterval):
+		case now := <-ticker.C:
+			log.Debugf("Starting controller cycle at %v", now)
 		case <-ctx.Done():
 			return
 		}