container stats: ignore context.Canceled errors

we often receive context.Canceled errors when a container exits during
docker stats collection.

there is a benign race condition where if we process the error first
before processing that the context is "Done", then we will log this
canceled error as a warning message here:
https://github.com/aws/amazon-ecs-agent/blob/5be7aa08bed215a557f48c16d8201ad3db59a9be/agent/stats/container.go#L118-L122

this change ignores these context.Canceled errors so that we don't log
them. This will eliminate log messages that look like this when a
container exits:

level=warn time=2020-12-25T07:51:33Z msg="Error encountered processing metrics stream from docker, this may affect cloudwatch metric accuracy: DockerGoClient: Unable to decode stats for container REDACTED: context canceled" module=container.go

agent/dockerclient/dockerapi/docker_client.go

@@ -1393,6 +1393,10 @@ func (dg *dockerGoClient) Stats(ctx context.Context, id string, inactivityTimeou
 			stream := true
 			resp, err = client.ContainerStats(subCtx, id, stream)
 			if err != nil {
+				if errors.Is(err, context.Canceled) {
+					// ignore context cancelled errors, which indicate container has exited already
+					return
+				}
 				errC <- fmt.Errorf("DockerGoClient: Unable to retrieve stats for container %s: %v", id, err)
 				return
 			}
@@ -1408,6 +1412,10 @@ func (dg *dockerGoClient) Stats(ctx context.Context, id string, inactivityTimeou
 			data := new(types.StatsJSON)
 			for err := decoder.Decode(data); err != io.EOF; err = decoder.Decode(data) {
 				if err != nil {
+					if errors.Is(err, context.Canceled) {
+						// ignore context cancelled errors, which indicate container has exited already
+						return
+					}
 					errC <- fmt.Errorf("DockerGoClient: Unable to decode stats for container %s: %v", id, err)
 					return
 				}
@@ -1466,6 +1474,10 @@ func getContainerStatsNotStreamed(client sdkclient.Client, ctx context.Context,
 	response := make(chan statsResponse, 1)
 	go func() {
 		stats, err := client.ContainerStats(ctxWithTimeout, id, false)
+		if errors.Is(err, context.Canceled) {
+			// ignore context cancelled errors, which indicate container has exited already
+			err = nil
+		}
 		response <- statsResponse{stats, err}
 	}()
 	select {