Today submitting an unbatched `ClusterStateUpdateTask` to the `MasterService` is deprecated, and devs are encouraged to write their own batching executor instead. In principle this makes it possible to combine updates without incurring the costs of building a whole new `ClusterState` on each iteration, but in practice most implementations don't work at a lower level and just iteratively update a `ClusterState`. Moreover creating new `ClusterState` instances is nothing like as expensive as once it was.

We should accept this and provide some friendlier utilities for writing batched cluster state updates, ideally leaving the developer to express the desired per-task update as a `ClusterState -> ClusterState` operator.
x-pack/plugin/rollup/src/main/java/org/elasticsearch/xpack/downsample/TransportRollupAction.java
final MapperService mapperService = indicesService.createIndexMapperServiceForValidation(sourceIndexMetadata);
            final CompressedXContent sourceIndexCompressedXContent = new CompressedXContent(sourceIndexMappings);
            mapperService.merge(MapperService.SINGLE_MAPPING_NAME, sourceIndexCompressedXContent, MapperService.MergeReason.INDEX_TEMPLATE);
            
            validateDownsamplingInterval(mapperService, request.getDownsampleConfig());
            final List<String> dimensionFields = new ArrayList<>();
            final List<String> metricFields = new ArrayList<>();
            final List<String> labelFields = new ArrayList<>();
            final TimeseriesFieldTypeHelper helper = new TimeseriesFieldTypeHelper.Builder(mapperService).build(
                request.getDownsampleConfig().getTimestampField()
            );
            MappingVisitor.visitMapping(sourceIndexMappings, (field, mapping) -> {
                if (helper.isTimeSeriesDimension(field, mapping)) {
                    dimensionFields.add(field);
                } else if (helper.isTimeSeriesMetric(field, mapping)) {
                    metricFields.add(field);
                } else if (helper.isTimeSeriesLabel(field, mapping)) {
                    labelFields.add(field);
                }
            });
            ActionRequestValidationException validationException = new ActionRequestValidationException();
            if (dimensionFields.isEmpty()) {
                validationException.addValidationError("Index [" + sourceIndexName + "] does not contain any dimension fields");
            }
            if (metricFields.isEmpty()) {
                validationException.addValidationError("Index [" + sourceIndexName + "] does not contain any metric fields");
            }
            if (validationException.validationErrors().isEmpty() == false) {
                listener.onFailure(validationException);
                return;