Fetch container stats in parallel (elastic#3127)

Currently fetching container stats is very slow as each request takes up to 2 seconds. To improve the fetching time if lots of containers are around, this creates the rrequests in parallel. The main downside is that this opens lots of connections. This fix should only temporary until the bulk api is available: moby/moby#25361

metricbeat/module/docker/docker.go

@@ -64,17 +64,35 @@ func FetchStats(client *docker.Client) ([]Stat, error) {
 		return nil, err
 	}
 
+	var wg sync.WaitGroup
+
 	containersList := []Stat{}
+	queue := make(chan Stat, 1)
+	wg.Add(len(containers))
+
 	for _, container := range containers {
-		// This is currently very inefficient as docker calculates the average for each request,
-		// means each request will take at least 2s: https://github.com/docker/docker/blob/master/cli/command/container/stats_helpers.go#L148
-		// Getting all stats at once is implemented here: https://github.com/docker/docker/pull/25361
-		containersList = append(containersList, exportContainerStats(client, &container))
+		go func(container docker.APIContainers) {
+			queue <- exportContainerStats(client, &container)
+		}(container)
 	}
 
+	go func() {
+		for container := range queue {
+			containersList = append(containersList, container)
+			wg.Done()
+		}
+	}()
+
+	wg.Wait()
+
 	return containersList, err
 }
 
+// exportContainerStats loads stats for the given container
+//
+// This is currently very inefficient as docker calculates the average for each request,
+// means each request will take at least 2s: https://github.com/docker/docker/blob/master/cli/command/container/stats_helpers.go#L148
+// Getting all stats at once is implemented here: https://github.com/docker/docker/pull/25361
 func exportContainerStats(client *docker.Client, container *docker.APIContainers) Stat {
 	var wg sync.WaitGroup
 	var event Stat