In some contexts is unclear why an autoscaling policy decides to request
more resources, i.e. if there's an unassigned shard that remains unassigned
even after adding more resources. It would be help to have more information
around the decisions taken by the deciders so it is easier to understand and
track them.
x-pack/plugin/autoscaling/src/main/java/org/elasticsearch/xpack/autoscaling/storage/ReactiveStorageDeciderService.java
ClusterInfo info,
            SnapshotShardSizeInfo shardSizeInfo,
            Set<DiscoveryNode> nodes,
            Set<DiscoveryNodeRole> roles
        ) {
            this.state = state;
            this.allocationDeciders = allocationDeciders;
            this.diskThresholdSettings = diskThresholdSettings;
            this.info = info;
            this.shardSizeInfo = shardSizeInfo;
            this.nodes = nodes;
            this.nodeIds = nodes.stream().map(DiscoveryNode::getId).collect(Collectors.toSet());
            this.nodeTierPredicate = nodes::contains;
            this.roles = roles;
        }
        public long storagePreventsAllocation() {
            RoutingAllocation allocation = new RoutingAllocation(allocationDeciders, state, info, shardSizeInfo, System.nanoTime());
            return StreamSupport.stream(state.getRoutingNodes().unassigned().spliterator(), false)
                .filter(shard -> canAllocate(shard, allocation) == false)
                .filter(shard -> cannotAllocateDueToStorage(shard, allocation))
                .mapToLong(this::sizeOf)
                .sum();
        }
        public long storagePreventsRemainOrMove() {
            RoutingAllocation allocation = new RoutingAllocation(allocationDeciders, state, info, shardSizeInfo, System.nanoTime());
            List<ShardRouting> candidates = new LinkedList<>();
            for (RoutingNode routingNode : state.getRoutingNodes()) {
                for (ShardRouting shard : routingNode) {
                    if (shard.started()
                        && canRemainOnlyHighestTierPreference(shard, allocation) == false