### 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/main/java/org/elasticsearch/xpack/downsample/TransportDownsampleIndexerAction.java
DownsampleIndexerAction.Request,
    DownsampleIndexerAction.Response,
    DownsampleIndexerAction.ShardDownsampleRequest,
    DownsampleIndexerAction.ShardDownsampleResponse> {
    private final Client client;
    private final ClusterService clusterService;
    private final IndicesService indicesService;
    @Inject
    public TransportDownsampleIndexerAction(
        Client client,
        ClusterService clusterService,
        TransportService transportService,
        IndicesService indicesService,
        ActionFilters actionFilters,
        IndexNameExpressionResolver indexNameExpressionResolver
    ) {
        super(
            DownsampleIndexerAction.NAME,
            clusterService,
            transportService,
            actionFilters,
            indexNameExpressionResolver,
            DownsampleIndexerAction.Request::new,