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
String compositeJobId = "stop-restart-composite";
        Job.Builder compositeJob = createScheduledJob(compositeJobId);
        compositeJob.setAnalysisConfig(new AnalysisConfig.Builder(compositeJob.getAnalysisConfig()).setSummaryCountFieldName("doc_count"));
        putJob(compositeJob);
        openJob(compositeJobId);
        assertBusy(() -> assertEquals(getJobStats(compositeJobId).get(0).getState(), JobState.OPENED));
        AggregatorFactories.Builder aggs = new AggregatorFactories.Builder();
        aggs.addAggregator(
            AggregationBuilders.composite(
                "buckets",
                Collections.singletonList(
                    new DateHistogramValuesSourceBuilder("timebucket").fixedInterval(new DateHistogramInterval("1h")).field("time")
                )
                
            ).subAggregation(AggregationBuilders.max("time").field("time")).size(1)
        );
        DatafeedConfig compositeDatafeedConfig = createDatafeedBuilder(
            compositeJobId + "-datafeed",
            compositeJobId,
            Collections.singletonList(indexName)
        ).setParsedAggregations(aggs).setFrequency(TimeValue.timeValueHours(1)).build();
        putDatafeed(compositeDatafeedConfig);
        startDatafeed(compositeDatafeedConfig.getId(), 0L, null);
        
        assertBusy(() -> assertThat(getDataCounts(compositeJobId).getProcessedRecordCount(), greaterThan(0L)));
        stopDatafeed(compositeDatafeedConfig.getId());
        assertBusy(() -> assertThat(getJobStats(compositeJobId).get(0).getState(), is(oneOf(JobState.CLOSED, JobState.OPENED))));
        
        if (getJobStats(compositeJobId).get(0).getState().equals(JobState.OPENED)) {
            aggs = new AggregatorFactories.Builder();