Reduce the mmap minimum size

Reduce the initial mmap file size from 1MiB to 64KiB. This should reduce
the memory and disk overhead for larger numbers of multiprocess workers.

Signed-off-by: SuperQ <superq@gmail.com>

Author: SuperQ

prometheus_client/mmap_dict.py

@@ -3,7 +3,7 @@
 import os
 import struct
 
-_INITIAL_MMAP_SIZE = 1 << 20
+_INITIAL_MMAP_SIZE = 1 << 16
 _pack_integer_func = struct.Struct(b'i').pack
 _pack_double_func = struct.Struct(b'd').pack
 _unpack_integer = struct.Struct(b'i').unpack_from