pageserver: add supplementary branch usage stats (#8131)

## Problem

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

Closes: https://github.com/neondatabase/neon/issues/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 https://github.com/neondatabase/neon/pull/8245
than this PR is adding.

test_runner/fixtures/metrics.py

@@ -144,6 +144,8 @@ PAGESERVER_PER_TENANT_METRICS: Tuple[str, ...] = (
     "pageserver_smgr_query_seconds_bucket",
     "pageserver_smgr_query_seconds_count",
     "pageserver_smgr_query_seconds_sum",
+    "pageserver_archive_size",
+    "pageserver_pitr_history_size",
     "pageserver_storage_operations_seconds_count_total",
     "pageserver_storage_operations_seconds_sum_total",
     "pageserver_evictions_total",