Make stats gathering idempotent

CHANGELOG.md

@@ -30,6 +30,7 @@ New features and enhancements:
 	[#1768](https://github.com/weaveworks/scope/pull/1768)	
 - Eliminate stats log noise from stopped containers
 	[#1687](https://github.com/weaveworks/scope/pull/1687)
+	[#1798](https://github.com/weaveworks/scope/pull/1798)	
 - Hide uncontained/unmanaged by default
 	[#1694](https://github.com/weaveworks/scope/pull/1694)
 

probe/docker/container.go

@@ -181,7 +181,7 @@ func (c *container) StartGatheringStats() error {
 	defer c.Unlock()
 
 	if c.statsConn != nil {
-		return fmt.Errorf("already gather stats for container %s", c.container.ID)
+		return nil
 	}
 
 	go func() {

probe/docker/registry.go

@@ -360,7 +360,7 @@ func (r *registry) updateContainerState(containerID string, intendedState *strin
 	if r.collectStats {
 		if dockerContainer.State.Running {
 			if err := c.StartGatheringStats(); err != nil {
-				log.Errorf("Error gather stats for container: %s", containerID)
+				log.Errorf("Error gathering stats for container %s: %s", containerID, err)
 				return
 			}
 		} else {