### 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
private static final DateFormatter DATE_FORMATTER = DateFormatter.forPattern("yyyy-MM-dd'T'HH:mm:ss.SSSXXX");
    public static final String FIELD_TIMESTAMP = "@timestamp";
    public static final String FIELD_DIMENSION_1 = "dimension_kw";
    public static final String FIELD_DIMENSION_2 = "dimension_long";
    public static final String FIELD_NUMERIC_1 = "numeric_1";
    public static final String FIELD_NUMERIC_2 = "numeric_2";
    public static final String FIELD_AGG_METRIC = "agg_metric_1";
    public static final String FIELD_METRIC_LABEL_DOUBLE = "metric_label_double";
    public static final String FIELD_LABEL_DOUBLE = "label_double";
    public static final String FIELD_LABEL_INTEGER = "label_integer";
    public static final String FIELD_LABEL_KEYWORD = "label_keyword";
    public static final String FIELD_LABEL_TEXT = "label_text";
    public static final String FIELD_LABEL_BOOLEAN = "label_boolean";
    public static final String FIELD_LABEL_IPv4_ADDRESS = "label_ipv4_address";
    public static final String FIELD_LABEL_IPv6_ADDRESS = "label_ipv6_address";
    public static final String FIELD_LABEL_DATE = "label_date";
    public static final String FIELD_LABEL_UNMAPPED = "label_unmapped";
    public static final String FIELD_LABEL_KEYWORD_ARRAY = "label_keyword_array";
    public static final String FIELD_LABEL_DOUBLE_ARRAY = "label_double_array";
    public static final String FIELD_LABEL_AGG_METRIC = "label_agg_metric";
    private static final int MAX_DIM_VALUES = 5;
    private static final long MAX_NUM_BUCKETS = 10;
    private String sourceIndex, rollupIndex;
    private long startTime;
    private int docCount, numOfShards, numOfReplicas;
    private List<String> dimensionValues;
    @Override