fix: Configure python urllib3 connection pool size (#17323)

This was removed in #16802, but using a higher value than 1,
or at least making this configurable makes complete sense.

Without this, we get a lot of these log messages:

[ WARNING] Connection pool is full, discarding connection:

samples/client/echo_api/python/openapi_client/configuration.py

@@ -15,6 +15,7 @@
 
 import copy
 import logging
+import multiprocessing
 import sys
 import urllib3
 
@@ -164,6 +165,13 @@ conf = openapi_client.Configuration(
            Set this to the SNI value expected by the server.
         """
 
+        self.connection_pool_maxsize = multiprocessing.cpu_count() * 5
+        """urllib3 connection pool's maximum number of connections saved
+           per pool. urllib3 uses 1 connection as default value, but this is
+           not the best value when you are making a lot of possibly parallel
+           requests to the same host, which is often the case here.
+           cpu_count * 5 is used as default value to increase performance.
+        """
 
         self.proxy = None
         """Proxy URL

samples/client/echo_api/python/openapi_client/rest.py

@@ -88,6 +88,9 @@ class RESTClientObject:
         if configuration.socket_options is not None:
             addition_pool_args['socket_options'] = configuration.socket_options
 
+        if configuration.connection_pool_maxsize is not None:
+            addition_pool_args['maxsize'] = configuration.connection_pool_maxsize
+
         # https pool manager
         if configuration.proxy:
             if is_socks_proxy_url(configuration.proxy):