To gather metrics/stats about a specific sandbox instance, it'd be nice to
have a dedicated rpc for this. Due to the same "what kind of stats are going
to be returned" dilemma exists for sandboxes as well, I've re-used the metrics
type we have as the data field is just an `any`, leaving the metrics returned
entirely up to the shim author. For CRI usecases this will just be cgroup and
windows stats as that's all that's supported right now.
Signed-off-by: Danny Canter <danny@dcantah.dev>
