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
Job.Builder jobBuilder = new Job.Builder();
        jobBuilder.setId("lookback-job-with-rt-fields");
        jobBuilder.setAnalysisConfig(analysisConfig);
        jobBuilder.setDataDescription(dataDescription);
        putJob(jobBuilder);
        openJob(jobBuilder.getId());
        assertBusy(() -> assertEquals(getJobStats(jobBuilder.getId()).get(0).getState(), JobState.OPENED));
        DatafeedConfig.Builder dfBuilder = new DatafeedConfig.Builder(jobBuilder.getId() + "-datafeed", jobBuilder.getId());
        dfBuilder.setQueryDelay(TimeValue.timeValueSeconds(1));
        dfBuilder.setFrequency(TimeValue.timeValueSeconds(1));
        dfBuilder.setIndices(Collections.singletonList("data-1"));
        Map<String, Object> properties = new HashMap<>();
        properties.put("type", "long");
        properties.put("script", "emit(doc['time'].value.getDayOfWeekEnum().value)");
        Map<String, Object> fields = new HashMap<>();
        fields.put("day_of_week", properties);
        dfBuilder.setRuntimeMappings(fields);
        DatafeedConfig datafeedConfig = dfBuilder.build();
        putDatafeed(datafeedConfig);
        startDatafeed(datafeedConfig.getId(), 0L, now);
        assertBusy(() -> {
            DataCounts dataCounts = getDataCounts(jobBuilder.getId());
            assertThat(dataCounts.getProcessedRecordCount(), equalTo(numDocs));
            assertThat(dataCounts.getOutOfOrderTimeStampCount(), equalTo(0L));
            assertThat(dataCounts.getMissingFieldCount(), 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(jobBuilder.getId());