[watcher] Deprecate Prometheus datasource

Hello everyone, As discussed during the 2026.1 PTG[1], it was decided to deprecate the prometheus datasource[2] during this cycle, in favor of the aetos datasource[3]. The aetos datasource provides the same functionality as the prometheus one, but with the advantage of multi-tenancy and Kestone-based authentication. Let us know if there are any objections.  A patch to deprecate the prometheus datasource is proposed[4]. [1] https://etherpad.opendev.org/p/watcher-2026.1-ptg#L207 [2] https://docs.openstack.org/watcher/latest/datasources/prometheus.html [3] https://docs.openstack.org/watcher/latest/datasources/aetos.html [4] https://review.opendev.org/c/openstack/watcher/+/966672 Regards, Joan Francesc