Use-case: for T259831 i'll be performing maintenance on db2117 which is the only instance in the dump/vslow groups for s6 in codfw. This means i need to add another instance to those groups, but there's no simple way to see a mapping from hosts to groups, only the other way around.

An example of what i'm thinking of:

$ dbctl --scope codfw section s6 list-groups-by-instance™
db2076: api
db2087:3316: contributions,logpager,recentchanges,recentchangeslinked,watchlist
db2089:3316: contributions,logpager,recentchanges,recentchangeslinked,watchlist
db2114: api
db2117: dump,vslow
db2124:
db2129: