wsgi server: address family discovery is not quite right (#1006)

The code introduced to improve binding to an IPv6 address is based on
similar code in Python itself, but is missing some critical arguments to
the socket.getaddrinfo() call: in particular, the socket type must be
set to SOCK_STREAM, because we want a TCP connection; the AI_PASSIVE
flag should also be passed because we intend to use the result for
binding a listen socket rather than making an outbound connection.

Signed-off-by: Joshua M. Clulow <josh@sysmgr.org>

Author: jclulow

prometheus_client/exposition.py

@@ -154,7 +154,7 @@ def _get_best_family(address, port):
     # binding an ipv6 address is requested.
     # This function is based on what upstream python did for http.server
     # in https://github.com/python/cpython/pull/11767
-    infos = socket.getaddrinfo(address, port)
+    infos = socket.getaddrinfo(address, port, type=socket.SOCK_STREAM, flags=socket.AI_PASSIVE)
     family, _, _, _, sockaddr = next(iter(infos))
     return family, sockaddr[0]