contrib(kolla): add ExecStartPre socket-wait to drop-in

Kolla container units go active as soon as `docker start -a` runs,
before the daemons inside open their sockets. Wait up to 120s for
the NB and SB sockets so the exporter does not race them and trip
StartLimitBurst at boot.

Author: lucadelmonte

contrib/kolla-ansible/ovn-exporter-kolla.conf

@@ -13,3 +13,9 @@
 After=docker.service kolla-ovn_nb_db-container.service kolla-ovn_sb_db-container.service kolla-ovn_northd-container.service
 Requires=kolla-ovn_nb_db-container.service kolla-ovn_sb_db-container.service
 Wants=kolla-ovn_northd-container.service
+
+[Service]
+# Kolla container units go active as soon as `docker start -a` runs, before the
+# daemons inside open their sockets. Wait up to 120s for the NB and SB sockets
+# so the exporter does not race them and trip StartLimitBurst at boot.
+ExecStartPre=/bin/sh -c 'for i in $(seq 1 120); do [ -S /run/ovn/ovnnb_db.sock ] && [ -S /run/ovn/ovnsb_db.sock ] && exit 0; sleep 1; done; exit 1'