Mtime.Spantype t = spanval zero : spanval one : spanval max_span : spanval ns : spanval us : spanval ms : spanval s : spanval min : spanval hour : spanval day : spanval year : spanval to_uint64_ns : span -> int64val of_uint64_ns : int64 -> spanval of_float_ns : float -> span optionval to_float_ns : span -> float