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
.cluster()
            .prepareUpdateSettings()
            .setPersistentSettings(Settings.builder().putNull("logger.org.elasticsearch.xpack.ml.datafeed").build())
            .get();
        cleanUp();
    }
    public void testLookbackOnly() throws Exception {
        client().admin().indices().prepareCreate("data-1").setMapping("time", "type=date").get();
        long numDocs = randomIntBetween(32, 2048);
        long now = System.currentTimeMillis();
        long oneWeekAgo = now - 604800000;
        long twoWeeksAgo = oneWeekAgo - 604800000;
        indexDocs(logger, "data-1", numDocs, twoWeeksAgo, oneWeekAgo);
        client().admin().indices().prepareCreate("data-2").setMapping("time", "type=date").get();
        client().admin().cluster().prepareHealth("data-1", "data-2").setWaitForYellowStatus().get();
        long numDocs2 = randomIntBetween(32, 2048);
        indexDocs(logger, "data-2", numDocs2, oneWeekAgo, now);
        Job.Builder job = createScheduledJob("lookback-job");
        PutJobAction.Response putJobResponse = putJob(job);
        assertThat(putJobResponse.getResponse().getJobVersion(), equalTo(Version.CURRENT));
        openJob(job.getId());
        assertBusy(() -> assertEquals(getJobStats(job.getId()).get(0).getState(), JobState.OPENED));
        
        List<String> indices = Arrays.asList("data-*", "missing-*");
        DatafeedConfig datafeedConfig = createDatafeed(job.getId() + "-datafeed", job.getId(), indices);
        putDatafeed(datafeedConfig);
        startDatafeed(datafeedConfig.getId(), 0L, now);
        assertBusy(() -> {
            DataCounts dataCounts = getDataCounts(job.getId());
            assertThat(dataCounts.getProcessedRecordCount(), equalTo(numDocs + numDocs2));