### 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
field -> assertTrue(
                                    "Field [" + field + "] is not included in the source fields: " + originalFieldsList,
                                    originalFieldsList.contains(field)
                                )
                            );
                            Object originalLabelValue = originalHit.getDocumentFields().values().stream().toList().get(0).getValue();
                            Object rollupLabelValue = rollupHit.getDocumentFields().values().stream().toList().get(0).getValue();
                            Optional<Aggregation> labelAsMetric = nonTopHitsOriginalAggregations.stream()
                                .filter(agg -> agg.getName().equals("metric_" + rollupTopHits.getName()))
                                .findFirst();
                            
                            if (labelAsMetric.isPresent()) {
                                double metricValue = ((Max) labelAsMetric.get()).value();
                                assertEquals(metricValue, rollupLabelValue);
                                assertEquals(metricValue, originalLabelValue);
                            }
                        }
                    }
                }
            }
        });
    }
    @SuppressWarnings("unchecked")
    private void assertFieldMappings(
        DownsampleConfig config,
        Map<String, TimeSeriesParams.MetricType> metricFields,
        Map<String, Map<String, Object>> mappings
    ) {
        
        assertEquals(DateFieldMapper.CONTENT_TYPE, mappings.get(config.getTimestampField()).get("type"));