Add initial framework for stats

Signed-off-by: Michael Crosby <crosbymichael@gmail.com>

runtime/runtime.go

@@ -11,10 +11,13 @@ var (
 	ErrInvalidContainerType = errors.New("containerd: invalid container type for runtime")
 	ErrCheckpointNotExists  = errors.New("containerd: checkpoint does not exist for container")
 	ErrCheckpointExists     = errors.New("containerd: checkpoint already exists")
+	ErrContainerExited      = errors.New("containerd: container has exited")
 )
 
-// runtime handles containers, containers handle their own actions.
+// Runtime handles containers, containers handle their own actions
 type Runtime interface {
+	// Create creates a new container initialized but without it starting it
 	Create(id, bundlePath string, stdio *Stdio) (Container, error)
+	// StartProcess adds a new process to the container
 	StartProcess(Container, specs.Process, *Stdio) (Process, error)
 }