Function mdb_range_count_values 

pub unsafe extern "C" fn mdb_range_count_values(
    txn: *mut MDB_txn,
    dbi: MDB_dbi,
    key_low: *const MDB_val,
    key_high: *const MDB_val,
    key_flags: c_uint,
    out: *mut u64,
) -> c_int