Use mmap.PAGESIZE constant as value for first read.

This size can have a real impact on performance when a lot of files need
to be read.
The performance relies heavily on the file system cache so reading as
little pages as possible helps a lot on memory constrained systems.

Signed-off-by: Xavier Fernandez <xavier.fernandez@polyconseil.fr>

prometheus_client/mmap_dict.py

@@ -83,7 +83,7 @@ def read_all_values_from_file(filename):
         with open(filename, 'rb') as infp:
             # Read the first block of data, including the first 4 bytes which tell us
             # how much of the file (which is preallocated to _INITIAL_MMAP_SIZE bytes) is occupied.
-            data = infp.read(65535)
+            data = infp.read(mmap.PAGESIZE)
             used = _unpack_integer(data, 0)[0]
             if used > len(data):  # Then read in the rest, if needed.
                 data += infp.read(used - len(data))