val lock_exn : lock_path:string -> metadata:string -> [ `We_took_it of t | `Somebody_else_took_it of string Core.Or_error.t ]metadata should include some information to help the user identify the lock holder. Usually it's the pid of the holder, but if you use this across a fork or take the lock multiple times in the same program, then some extra information could be useful. This string will be saved as the target of a (usually dangling) symbolic link at path lock_path.
`Somebody_else_took_it returns the metadata of the process who took it or an error if that can't be determined (for example: they released the lock by the time we tried to inspect it)
Raises an exception if taking the lock fails for any reason other than somebody else holding the lock.
val unlock_exn : t -> unit