Log warning when a registry is added to composite after meter registration

[shakuzen]

I also think we should pursue adding a warning log so that users who can do all setup before registering metrics can avoid surprises like this and others (such as the values being recorded before a non-composite registry is added being dropped).

Originally posted by @shakuzen in #6846 (review)

See also #6811 (comment)

[shakuzen]

It could be done in a separate issue, but probably there is some documentation update that should accompany this. Perhaps the warning logged could link to the documentation (although that's potentially fragile as the link needs to always work going forward).