This patch removes the usage of eventlet in the Metadata Agent. The new
metadata proxy is based on the implementation done in [1]. This new
new implementation mimics the implementation of the
``UnixDomainWSGIServer`` class. The server listens to a file socket
that is populated by the HAProxy instance running inside the
network metadata namespace. The HAProxy writes the requests of the
virtual machines on the Unix file socket and the metadata proxy
sends the requests to the Nova metadata server.
This implementation doesn't use ``oslo.services`` to spawn the
processes or the local threads depending on the ``metadata_workers``
configuration variable. Right now only the embedded form (local thread)
is implemented (metadata_workers=0, the default value). Future
implementations will enable again this configuration variable.
[1]https://review.opendev.org/c/openstack/neutron/+/937545
Closes-Bug: #2099896
Change-Id: Ifadef192ef766a4f291643868686a98b591ddb40
bc0741e689