Fix stats collector spinning CPU if no stats are collected

Commit fd0e24b7189374e0fe7c55b6d26ee916d3ee1655 changed
the stats collection loop to use a `sleep()` instead
of `time.Tick()` in the for-loop.

This change caused a regression in situations where
no stats are being collected, or an error is hit
in the loop (in which case the loop would `continue`,
and the `sleep()` is not hit).

This patch puts the sleep at the start of the loop
to guarantee it's always hit.

This will delay the sampling, which is similar to the
behavior before fd0e24b7189374e0fe7c55b6d26ee916d3ee1655.

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>
Upstream-commit: 481b8e54b45955e40075f49a9af321afce439320
Component: engine

components/engine/daemon/stats/collector.go

@@ -91,6 +91,10 @@ func (s *Collector) Run() {
 	var pairs []publishersPair
 
 	for {
+		// Put sleep at the start so that it will always be hit,
+		// preventing a tight loop if no stats are collected.
+		time.Sleep(s.interval)
+
 		// it does not make sense in the first iteration,
 		// but saves allocations in further iterations
 		pairs = pairs[:0]
@@ -141,8 +145,6 @@ func (s *Collector) Run() {
 				logrus.Errorf("collecting stats for %s: %v", pair.container.ID, err)
 			}
 		}
-
-		time.Sleep(s.interval)
 	}
 }