revert: remove access-frequency ILM tiering (#60)

Reverse the first-parent diff of a3df317ae0,
including the feature branch compatibility and mover follow-up fixes.
Retain the independent multi-pool correctness fixes from #178 and migrate
their shared test fixture away from access-tier code.

Tolerate retired ILM keys and XML, read old v9 statistics while writing v8,
and document migration without moving objects or rewriting their metadata.
Include regression coverage using a historical scanner/writer v9 fixture.

Signed-off-by: Feng Ruohang <rh@vonng.com>

cmd/metrics-v3.go

@@ -392,17 +392,6 @@ func newMetricGroups(r *prometheus.Registry) *metricsV3Collection {
 			ilmTransitionActiveTasksMD,
 			ilmTransitionPendingTasksMD,
 			ilmTransitionMissedImmediateTasksMD,
-			ilmAccessTierActiveTasksMD,
-			ilmAccessTierPendingTasksMD,
-			ilmAccessTierPromotionsTotalMD,
-			ilmAccessTierDemotionsTotalMD,
-			ilmAccessTierBytesMovedTotalMD,
-			ilmAccessTierFailuresTotalMD,
-			ilmAccessTierSkippedWatermarkMD,
-			ilmAccessTierSkippedMaxSizeMD,
-			ilmAccessTierSkippedQuotaMD,
-			ilmAccessTierHotBytesMD,
-			ilmAccessTierSamplesDroppedMD,
 			ilmVersionsScannedMD,
 		},
 		loadILMMetrics,