[v26.1.x] iceberg: bring back in-manifest stats

[vbotbuildovich]

Backport #31332 to branch v26.1.x. This issue was created as a placeholder because the original PR's commit(s) could not be automatically cherry-picked.
Here are the commands to execute:

git checkout v26.1.x
git checkout -b manual-backport-31332-v26.1.x-885
git cherry-pick -x ddb4ace072 fcad9e1362 f3cfe07a0e 9a98ac5ee1 e12338af5b cb7ebf5905 5497e8fc15 3eaf8279bc babc950f87 5e64e28bf6

git push origin manual-backport-31332-v26.1.x-885
gh pr create \
  --title "[v26.1.x] iceberg: bring back in-manifest stats" \
  --base "v26.1.x" \
  --label "kind/backport" \
  --head "manual-backport-31332-v26.1.x-885" \
  --draft \
  --repo "redpanda-data/redpanda" \
  --milestone "v26.1.x-next" \
  --body "Backport of PR https://github.com/redpanda-data/redpanda/pull/31332 "