### 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
);
        assertNotNull(sourceSettings.get(IndexSettings.TIME_SERIES_END_TIME.getKey()));
        assertNotNull(rollupSettings.get(IndexSettings.TIME_SERIES_END_TIME.getKey()));
        assertEquals(
            sourceSettings.get(IndexSettings.TIME_SERIES_END_TIME.getKey()),
            rollupSettings.get(IndexSettings.TIME_SERIES_END_TIME.getKey())
        );
        assertNotNull(sourceSettings.get(IndexMetadata.INDEX_ROUTING_PATH.getKey()));
        assertNotNull(rollupSettings.get(IndexMetadata.INDEX_ROUTING_PATH.getKey()));
        assertEquals(
            sourceSettings.get(IndexMetadata.INDEX_ROUTING_PATH.getKey()),
            rollupSettings.get(IndexMetadata.INDEX_ROUTING_PATH.getKey())
        );
        assertNotNull(sourceSettings.get(IndexMetadata.SETTING_NUMBER_OF_SHARDS));
        assertNotNull(rollupSettings.get(IndexMetadata.SETTING_NUMBER_OF_SHARDS));
        assertEquals(
            sourceSettings.get(IndexMetadata.SETTING_NUMBER_OF_SHARDS),
            rollupSettings.get(IndexMetadata.SETTING_NUMBER_OF_SHARDS)
        );
        assertNotNull(sourceSettings.get(IndexMetadata.SETTING_NUMBER_OF_REPLICAS));
        assertNotNull(rollupSettings.get(IndexMetadata.SETTING_NUMBER_OF_REPLICAS));
        assertEquals(
            sourceSettings.get(IndexMetadata.SETTING_NUMBER_OF_REPLICAS),
            rollupSettings.get(IndexMetadata.SETTING_NUMBER_OF_REPLICAS)
        );
        assertEquals("true", rollupSettings.get(IndexMetadata.SETTING_BLOCKS_WRITE));
        assertEquals(sourceSettings.get(IndexMetadata.SETTING_INDEX_HIDDEN), rollupSettings.get(IndexMetadata.SETTING_INDEX_HIDDEN));
        if (sourceSettings.keySet().contains(MapperService.INDEX_MAPPING_TOTAL_FIELDS_LIMIT_SETTING.getKey())) {
            assertNotNull(indexSettingsResp.getSetting(rollupIndex, MapperService.INDEX_MAPPING_TOTAL_FIELDS_LIMIT_SETTING.getKey()));
        }