Filesystem errors in bb-storage are recorded as "Unknown" in prometheus

[lijunsong]

When the underlying storage is full (due to a bad configuration), bb-storage throws the following errors back to worker.

 "status":{"code":2, "message":"Failed to store cached action result: Shard 1: Backend A: no space left on device"}}

However, from prometheus, the storage returns "Unknown" grpc code. Same for frontend write call (see the screenshot below).