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
assertThat(dataCounts.getOutOfOrderTimeStampCount(), equalTo(0L));
            GetDatafeedsStatsAction.Request request = new GetDatafeedsStatsAction.Request(datafeedConfig.getId());
            GetDatafeedsStatsAction.Response response = client().execute(GetDatafeedsStatsAction.INSTANCE, request).actionGet();
            assertThat(response.getResponse().results().get(0).getDatafeedState(), equalTo(DatafeedState.STOPPED));
        }, 60, TimeUnit.SECONDS);
        waitUntilJobIsClosed(job.getId());
    }
    public void testLookbackOnlyDataStream() throws Exception {
        String mapping = """
            {
                  "properties": {
                    "time": {
                      "type": "date"
                    },        "@timestamp": {
                      "type": "date"
                    }      }
                }""";
        createDataStreamAndTemplate("datafeed_data_stream", mapping);
        long numDocs = randomIntBetween(32, 2048);
        long now = System.currentTimeMillis();
        long oneWeekAgo = now - 604800000;
        long twoWeeksAgo = oneWeekAgo - 604800000;
        indexDocs(logger, "datafeed_data_stream", numDocs, twoWeeksAgo, oneWeekAgo);
        client().admin().cluster().prepareHealth("datafeed_data_stream").setWaitForYellowStatus().get();
        Job.Builder job = createScheduledJob("lookback-data-stream-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));
        DatafeedConfig datafeedConfig = createDatafeed(