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/test/java/org/elasticsearch/xpack/autoscaling/storage/ReactiveStorageDeciderDecisionTests.java
int round = 0;
        while (lastState != state && round < maxRounds) {
            long numPrevents = numAllocatableSubjectShards();
            assert round != 0 || numPrevents > 0 : "must have shards that can be allocated on first round";
            verify(ReactiveStorageDeciderService.AllocationState::storagePreventsAllocation, numPrevents, mockCanAllocateDiskDecider);
            verify(
                ReactiveStorageDeciderService.AllocationState::storagePreventsAllocation,
                0,
                mockCanAllocateDiskDecider,
                CAN_ALLOCATE_NO_DECIDER
            );
            verify(ReactiveStorageDeciderService.AllocationState::storagePreventsAllocation, 0);
            
            verify(
                moveToCold(allIndices()),
                ReactiveStorageDeciderService.AllocationState::storagePreventsAllocation,
                state.getRoutingNodes().unassigned().size(),
                DiscoveryNodeRole.DATA_COLD_NODE_ROLE
            );
            verify(ReactiveStorageDeciderService.AllocationState::storagePreventsAllocation, 0, DiscoveryNodeRole.DATA_COLD_NODE_ROLE);
            if (numPrevents > 0) {
                verifyScale(numPrevents, "not enough storage available, needs " + numPrevents + "b", mockCanAllocateDiskDecider);
            } else {
                verifyScale(0, "storage ok", mockCanAllocateDiskDecider);
            }
            verifyScale(0, "storage ok", mockCanAllocateDiskDecider, CAN_ALLOCATE_NO_DECIDER);
            verifyScale(0, "storage ok");
            verifyScale(addDataNodes(DATA_HOT_NODE_ROLE, "additional", state, hotNodes), 0, "storage ok", mockCanAllocateDiskDecider);
            lastState = state;
            startRandomShards();