### Description

This is related to #96757.

Right now downsampling uses a thread pool with a single running thread (disallowing concurrent downsampling operations at the transport level) and an unbounded queue of waiting threads, queueing and waiting to be executed. We need to explore options around using multiple running threads here.
x-pack/plugin/rollup/src/test/java/org/elasticsearch/xpack/downsample/DownsampleActionSingleNodeTests.java
final MapperService mapperService = indicesService.createIndexMapperServiceForValidation(indexMetadata);
        final CompressedXContent sourceIndexCompressedXContent = new CompressedXContent(sourceIndexMappings);
        mapperService.merge(MapperService.SINGLE_MAPPING_NAME, sourceIndexCompressedXContent, MapperService.MergeReason.INDEX_TEMPLATE);
        TimeseriesFieldTypeHelper helper = new TimeseriesFieldTypeHelper.Builder(mapperService).build(config.getTimestampField());
        Map<String, TimeSeriesParams.MetricType> metricFields = new HashMap<>();
        Map<String, String> labelFields = new HashMap<>();
        MappingVisitor.visitMapping(sourceIndexMappings, (field, fieldMapping) -> {
            if (helper.isTimeSeriesMetric(field, fieldMapping)) {
                metricFields.put(field, TimeSeriesParams.MetricType.fromString(fieldMapping.get(TIME_SERIES_METRIC_PARAM).toString()));
            } else if (helper.isTimeSeriesLabel(field, fieldMapping)) {
                labelFields.put(field, fieldMapping.get("type").toString());
            }
        });
        assertRollupIndexAggregations(sourceIndex, rollupIndex, config, metricFields, labelFields);
        GetIndexResponse indexSettingsResp = client().admin().indices().prepareGetIndex().addIndices(sourceIndex, rollupIndex).get();
        assertRollupIndexSettings(sourceIndex, rollupIndex, indexSettingsResp);
        Map<String, Map<String, Object>> mappings = (Map<String, Map<String, Object>>) indexSettingsResp.getMappings()
            .get(rollupIndex)
            .getSourceAsMap()
            .get("properties");
        assertFieldMappings(config, metricFields, mappings);
        GetMappingsResponse indexMappings = client().admin()
            .indices()
            .getMappings(new GetMappingsRequest().indices(rollupIndex, sourceIndex))
            .actionGet();
        Map<String, String> rollupIndexProperties = (Map<String, String>) indexMappings.mappings()
            .get(rollupIndex)
            .sourceAsMap()
            .get("properties");
        Map<String, String> sourceIndexCloneProperties = (Map<String, String>) indexMappings.mappings()