neutron-metadata-agent dies with "error: [Errno 24] Too many open files"

Issue

In our environment neutron-metadata-agent stops working after a while:

neutron-metadata-agent.service - OpenStack Neutron Metadata Agent
   Loaded: loaded (/usr/lib/systemd/system/neutron-metadata-agent.service; enabled)
   Active: inactive (dead) since Mo 2014-12-08 16:58:48 CET; 18h ago
  Process: 27686 ExecStart=/usr/bin/neutron-metadata-agent --config-file /usr/share/neutron/neutron-dist.conf --config-file /etc/neutron/neutron.conf --config-file /etc/neutron/metadata_agent.ini --log-file /var/log/neutron/metadata-agent.log (code=exited, status=0/SUCCESS)
 Main PID: 27686 (code=exited, status=0/SUCCESS)
   CGroup: /system.slice/neutron-metadata-agent.service

Dez 08 16:57:40 neutron.dedicated.network.de neutron-metadata-agent[27686]: log=logging.WritableLogger(logger))
Dez 08 16:57:40 neutron.dedicated.network.de neutron-metadata-agent[27686]: File "/usr/lib/python2.7/site-packages/eventlet...ver
Dez 08 16:57:40 neutron.dedicated.network.de neutron-metadata-agent[27686]: client_socket = sock.accept()
Dez 08 16:57:40 neutron.dedicated.network.de neutron-metadata-agent[27686]: File "/usr/lib/python2.7/site-packages/eventlet...ept
Dez 08 16:57:40 neutron.dedicated.network.de neutron-metadata-agent[27686]: res = socket_accept(fd)
Dez 08 16:57:40 neutron.dedicated.network.de neutron-metadata-agent[27686]: File "/usr/lib/python2.7/site-packages/eventlet...ept
Dez 08 16:57:40 neutron.dedicated.network.de neutron-metadata-agent[27686]: return descriptor.accept()
Dez 08 16:57:40 neutron.dedicated.network.de neutron-metadata-agent[27686]: File "/usr/lib64/python2.7/socket.py", line 202...ept
Dez 08 16:57:40 neutron.dedicated.network.de neutron-metadata-agent[27686]: sock, addr = self._sock.accept()
Dez 08 16:57:40 neutron.dedicated.network.de neutron-metadata-agent[27686]: error: [Errno 24] Too many open files
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