### 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
}
    public void testRollupEmptyIndex() throws IOException {
        DownsampleConfig config = new DownsampleConfig(randomInterval());
        
        prepareSourceIndex(sourceIndex);
        rollup(sourceIndex, rollupIndex, config);
        assertRollupIndex(sourceIndex, rollupIndex, config);
    }
    public void testRollupIndexWithNoMetrics() throws IOException {
        
        String sourceIndex = "no-metrics-idx-" + randomAlphaOfLength(5).toLowerCase(Locale.ROOT);
        client().admin()
            .indices()
            .prepareCreate(sourceIndex)
            .setSettings(
                indexSettings(numOfShards, numOfReplicas).put(IndexSettings.MODE.getKey(), IndexMode.TIME_SERIES)
                    .putList(IndexMetadata.INDEX_ROUTING_PATH.getKey(), List.of(FIELD_DIMENSION_1))
                    .put(IndexSettings.TIME_SERIES_START_TIME.getKey(), Instant.ofEpochMilli(startTime).toString())
                    .put(IndexSettings.TIME_SERIES_END_TIME.getKey(), "2106-01-08T23:40:53.384Z")
            )
            .setMapping(
                FIELD_TIMESTAMP,
                "type=date",
                FIELD_DIMENSION_1,
                "type=keyword,time_series_dimension=true",
                FIELD_DIMENSION_2,
                "type=long,time_series_dimension=true"
            )
            .get();
        DownsampleConfig config = new DownsampleConfig(randomInterval());