feat(tier): report cluster tier stats and count tier requests (#7110)

`GET /v3/tier-stats` answered from whichever process received the
request, returning that node's rolling 24-hour transition counters as
if they were cluster totals, and the `TierRequestsSuccess` and
`TierRequestsFailure` metric names had no producer at all.

The body now separates the two quantities a tier carries. Stored
inventory comes from the persisted scanner usage snapshot, which is
already cluster-wide; rolling activity is summed over every member
through a new read-only `TierDailyStats` peer RPC. Rings are merged
rather than added, so an idle node's expired hours age out, and each
node counts only its own committed transitions, so a retry is counted
once. Coverage travels with the numbers: `activity.status` names the
reporting members and the ones that could not be asked, timed out, or
answered with a ring this build refuses to merge, and per-tier
inventory is absent rather than zero when the snapshot has no
accounting. The version 1 body stays reachable at `?format=legacy`.

Tier request counters are recorded at the two seams every remote
request passes through, so a new provider is counted by construction,
with a closed operation/outcome label set that can never grow a tier
name, endpoint or object key.

Closes rustfs/backlog#2207

Co-authored-by: cxymds <cxymds@gmail.com>

docs/architecture/README.md

@@ -48,6 +48,7 @@ Required headings and strings in these files are asserted by `scripts/check_arch
 | [config-model-boundary-adr.md](config-model-boundary-adr.md) | touching the server-config model (`Config`, `KV`, `KVS`) or its persistence, or asking which crate owns which part of server configuration |
 | [admin-route-action-snapshot.md](admin-route-action-snapshot.md) | adding, moving, or re-authorizing an admin route and needing to know where the route → handler → `AdminAction` contract is enforced |
 | [kms-bulk-rekey-contract.md](kms-bulk-rekey-contract.md) | changing the bulk envelope re-wrap sweep, its admin endpoints, the re-wrap primitive, or which objects a rekey may touch |
+| [tier-stats-contract.md](tier-stats-contract.md) | changing what `GET /rustfs/admin/v3/tier-stats` returns, adding a tier accounting source, or wiring a metric to a remote tier request |
 
 ## Support and compatibility matrices (release-facing, keep current)