After applying the fix in bionic-proposed, 
only one compute node exhibited the logs mentioned in #5
2021-01-12 06:48:49.848 52569 ERROR neutron.agent.ovn.metadata.server [-] Unexpected error.: AttributeError: 'MetadataProxyHandler' object has no attribute 'sb_idl'


sb_idl is never initialised as the connection timed out during initialisation phase.

See error log:

2021-04-15 22:30:08.542 69188 ERROR neutron.plugins.ml2.drivers.ovn.mech_driver.ovsdb.impl_idl_ovn [-] OVS database connection to OVN_Southbound failed with error: 'Timeout'. Verify that the OVS and OVN services are available and that the 'ovn_nb_connection' and 'ovn_sb_connection' configuration options are correct.: Exception: Timeout
2021-04-15 22:30:08.542 69188 ERROR neutron.plugins.ml2.drivers.ovn.mech_driver.ovsdb.impl_idl_ovn Traceback (most recent call last):
2021-04-15 22:30:08.542 69188 ERROR neutron.plugins.ml2.drivers.ovn.mech_driver.ovsdb.impl_idl_ovn   File "/usr/lib/python3/dist-packages/neutron/plugins/ml2/drivers/ovn/mech_driver/ovsdb/impl_idl_ovn.py", line 67, in start_connection
2021-04-15 22:30:08.542 69188 ERROR neutron.plugins.ml2.drivers.ovn.mech_driver.ovsdb.impl_idl_ovn     self.ovsdb_connection.start()
2021-04-15 22:30:08.542 69188 ERROR neutron.plugins.ml2.drivers.ovn.mech_driver.ovsdb.impl_idl_ovn   File "/usr/lib/python3/dist-packages/ovsdbapp/backend/ovs_idl/connection.py", line 79, in start
2021-04-15 22:30:08.542 69188 ERROR neutron.plugins.ml2.drivers.ovn.mech_driver.ovsdb.impl_idl_ovn     idlutils.wait_for_change(self.idl, self.timeout)
2021-04-15 22:30:08.542 69188 ERROR neutron.plugins.ml2.drivers.ovn.mech_driver.ovsdb.impl_idl_ovn   File "/usr/lib/python3/dist-packages/ovsdbapp/backend/ovs_idl/idlutils.py", line 173, in wait_for_change
2021-04-15 22:30:08.542 69188 ERROR neutron.plugins.ml2.drivers.ovn.mech_driver.ovsdb.impl_idl_ovn     raise Exception("Timeout")  # TODO(twilson) use TimeoutException?
2021-04-15 22:30:08.542 69188 ERROR neutron.plugins.ml2.drivers.ovn.mech_driver.ovsdb.impl_idl_ovn Exception: Timeout
2021-04-15 22:30:08.542 69188 ERROR neutron.plugins.ml2.drivers.ovn.mech_driver.ovsdb.impl_idl_ovn
2021-04-15 22:30:08.544 69188 ERROR neutron_lib.callbacks.manager [-] Error during notification for neutron.agent.ovn.metadata.server.MetadataProxyHandler.post_fork_initialize-476074 process, after_init: neutron.plugins.ml2.drivers.ovn.mech_driver.ovsdb.impl_idl_ovn.OvsdbConnectionUnavailable: OVS database connection to OVN_Southbound failed with error: 'Timeout'. Verify that the OVS and OVN services are available and that the 'ovn_nb_connection' and 'ovn_sb_connection' configuration options are correct.
2021-04-15 22:30:08.544 69188 ERROR neutron_lib.callbacks.manager Traceback (most recent call last):
2021-04-15 22:30:08.544 69188 ERROR neutron_lib.callbacks.manager   File "/usr/lib/python3/dist-packages/neutron/plugins/ml2/drivers/ovn/mech_driver/ovsdb/impl_idl_ovn.py", line 67, in start_connection
2021-04-15 22:30:08.544 69188 ERROR neutron_lib.callbacks.manager     self.ovsdb_connection.start()
2021-04-15 22:30:08.544 69188 ERROR neutron_lib.callbacks.manager   File "/usr/lib/python3/dist-packages/ovsdbapp/backend/ovs_idl/connection.py", line 79, in start
2021-04-15 22:30:08.544 69188 ERROR neutron_lib.callbacks.manager     idlutils.wait_for_change(self.idl, self.timeout)
2021-04-15 22:30:08.544 69188 ERROR neutron_lib.callbacks.manager   File "/usr/lib/python3/dist-packages/ovsdbapp/backend/ovs_idl/idlutils.py", line 173, in wait_for_change
2021-04-15 22:30:08.544 69188 ERROR neutron_lib.callbacks.manager     raise Exception("Timeout")  # TODO(twilson) use TimeoutException?
2021-04-15 22:30:08.544 69188 ERROR neutron_lib.callbacks.manager Exception: Timeout