### 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
DownsampleConfig config = new DownsampleConfig(randomInterval());
        SourceSupplier sourceSupplier = () -> XContentFactory.jsonBuilder()
            .startObject()
            .field(FIELD_TIMESTAMP, randomDateForInterval(config.getInterval()))
            .field(FIELD_DIMENSION_1, randomAlphaOfLength(1))
            .field(FIELD_NUMERIC_1, randomDouble())
            .endObject();
        bulkIndex(sourceSupplier);
        prepareSourceIndex(sourceIndex);
        IndicesService indexServices = getInstanceFromNode(IndicesService.class);
        Index srcIndex = resolveIndex(sourceIndex);
        IndexService indexService = indexServices.indexServiceSafe(srcIndex);
        int shardNum = randomIntBetween(0, numOfShards - 1);
        IndexShard shard = indexService.getShard(shardNum);
        RollupShardTask task = new RollupShardTask(
            randomLong(),
            "rollup",
            "action",
            TaskId.EMPTY_TASK_ID,
            rollupIndex,
            config,
            emptyMap(),
            shard.shardId()
        );
        
        RollupShardIndexer indexer = new RollupShardIndexer(
            task,
            client(),
            indexService,