Problem

The metrics we have today aren't convenient for planning around the impact of timeline archival on costs.

Closes: #8108

Summary of changes

• Add metric pageserver_archive_size, which indicates the logical bytes of data which we would expect to write into an archived branch.
• Add metric pageserver_pitr_history_size, which indicates the distance between last_record_lsn and the PITR cutoff.

These metrics are somewhat temporary: when we implement #8088 and associated consumption metric changes, these will reach a final form. For now, an "archived" branch is just any branch outside of its parent's PITR window: later, archival will become an explicit state (which will usually correspond to falling outside the parent's PITR window).

The overall volume of timeline metrics is something to watch, but we are removing many more in #8245 than this PR is adding.

Checklist before requesting a review

• I have performed a self-review of my code.
• If it is a core feature, I have added thorough tests.
• Do we need to implement analytics? if so did you add the relevant metrics to the dashboard?
• If this PR requires public announcement, mark it with /release-notes label and add several sentences in this section.

Checklist before merging

• Do not forget to reformat commit message to not include the above checklist