wsgi server: allow binding to ipv6 address

Reproduce the fix in python3.8 http.server by doing a dns lookup and
using the correct address family

Fixes #567

Signed-off-by: Dominique Martinet <asmadeus@codewreck.org>

Author: martinetd

prometheus_client/exposition.py

@@ -143,9 +143,20 @@ class ThreadingWSGIServer(ThreadingMixIn, WSGIServer):
     daemon_threads = True
 
 
+def _get_best_family(*address):
+    infos = socket.getaddrinfo(
+        *address,
+        type=socket.SOCK_STREAM,
+        flags=socket.AI_PASSIVE,
+    )
+    family, type, proto, canonname, sockaddr = next(iter(infos))
+    return family, sockaddr[0]
+
+
 def start_wsgi_server(port, addr='', registry=REGISTRY):
     """Starts a WSGI server for prometheus metrics as a daemon thread."""
     app = make_wsgi_app(registry)
+    ThreadingWSGIServer.address_family, addr = _get_best_family(addr, port)
     httpd = make_server(addr, port, app, ThreadingWSGIServer, handler_class=_SilentHandler)
     t = threading.Thread(target=httpd.serve_forever)
     t.daemon = True