As per title, all hosts running role::prometheus should be on Bullseye

root@cumin1001:~# cumin 'P{C:prometheus} and not P{F:lsbdistcodename = buster}'
7 hosts will be targeted:
bast[3004,4002,5001].wikimedia.org,prometheus[2003-2004].codfw.wmnet,prometheus[1003-1004].eqiad.wmnet
DRY-RUN mode enabled, aborting

For bastions see also T243057: Move Prometheus off eqsin/ulsfo/esams bastions.

As part of T222113: prometheus: upgrade to >= 2.12 we have Prometheus for Bullseye, so we should be targeting Bullseye instead (unless there's a good reason not to). There will be Prometheus hardware coming in FY 21-22 too as a good occasion for the upgrade.