We should add relevant statistics for checkpoints in disaggregated storage.
For example, it could be worth adding statistics such as:

• the number of checkpoints that we picked up as a follower
• the number of checkpoints that we completed as a leader

This would be, among other things, useful in testing. For example, after WT-14498 deprecated checkpoint IDs, test_layered23 lost its ability to confirm that it picked up a new checkpoint (see #12188). We should use the new statistics to add back these checks, either as a part of this ticket or as a new ticket.