Since #87849 and #88534 it is possible that anomaly detection jobs and datafeeds will encounter the `aggregate_metric_double` type.

We need to investigate how this will affect certain aspects of their behaviour:

1. Specifying detector fields when the datafeed is using an aggregation
2. Specifying detector fields when the datafeed is not using an aggregation
3. Ability to switch a datafeed between using an aggregation and not using an aggregation without deleting and recreating the job
4. Correct identification of whether to use `_source` or doc values to get values from `aggregate_metric_double` fields
5. How to appropriately set `summary_count_field` when using `aggregate_metric_double`

Based on this investigation decide what changes we should make to either code or documentation.
x-pack/plugin/ml/qa/native-multi-node-tests/src/javaRestTest/java/org/elasticsearch/xpack/ml/integration/DatafeedJobsIT.java
client().admin().indices().prepareCreate("data").setMapping("time", "type=date").get();
        long numDocs = randomIntBetween(32, 2048);
        Instant now = Instant.now();
        indexDocs(logger, "data", numDocs, now.minus(Duration.ofDays(14)).toEpochMilli(), now.toEpochMilli());
        Job.Builder job = createScheduledJob("lookback-job-query-delay-updated");
        putJob(job);
        String datafeedId = "lookback-datafeed-query-delay-updated";
        DatafeedConfig datafeedConfig = createDatafeed(datafeedId, job.getId(), Collections.singletonList("data"));
        putDatafeed(datafeedConfig);
        openJob(job.getId());
        assertBusy(() -> assertEquals(getJobStats(job.getId()).get(0).getState(), JobState.OPENED));
        
        assertDatafeedStats(datafeedId, DatafeedState.STOPPED, job.getId(), equalTo(0L));
        startDatafeed(datafeedId, 0L, now.toEpochMilli());
        assertBusy(() -> {
            assertThat(getDataCounts(job.getId()).getProcessedRecordCount(), equalTo(numDocs));
            
            assertDatafeedStats(datafeedId, DatafeedState.STOPPED, job.getId(), greaterThan(0L));
        }, 60, TimeUnit.SECONDS);
        waitUntilJobIsClosed(job.getId());
        
        updateDatafeed(new DatafeedUpdate.Builder(datafeedId).setQueryDelay(TimeValue.timeValueSeconds(777)).build());
        
        assertDatafeedStats(datafeedId, DatafeedState.STOPPED, job.getId(), greaterThan(0L));
    }
    public void testStopAndRestartCompositeDatafeed() throws Exception {
        client().admin()
            .cluster()
            .prepareUpdateSettings()
            .setPersistentSettings(Settings.builder().put("logger.org.elasticsearch.xpack.ml.datafeed", "TRACE").build())