Stats.StatLoggerSourceSave stats between mutliple codex runs. Each logger saves a mapping string -> stat between various runs.
THE LOGGER MUST BE INSTANCIATED BEFORE THE STARTUP HOOK IS EXECUTED, an failwith will be triggered if that is not the case.
module S : sig ... endadd key stat Add a stat with the given key. If a stat is already present for this key, they are merged with S.combine key old_stat stat
Get the stat value for the key, None if the key is undefined.