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().cluster().prepareHealth(indexName).setWaitForYellowStatus().get();
        String scrollJobId = "stop-restart-scroll";
        Job.Builder scrollJob = createScheduledJob(scrollJobId);
        putJob(scrollJob);
        openJob(scrollJobId);
        assertBusy(() -> assertEquals(getJobStats(scrollJobId).get(0).getState(), JobState.OPENED));
        DatafeedConfig datafeedConfig = createDatafeedBuilder(scrollJobId + "-datafeed", scrollJobId, Collections.singletonList(indexName))
            .setChunkingConfig(ChunkingConfig.newManual(new TimeValue(1, TimeUnit.SECONDS)))
            .build();
        putDatafeed(datafeedConfig);
        startDatafeed(datafeedConfig.getId(), 0L, null);
        
        assertBusy(() -> assertThat(getDataCounts(scrollJobId).getProcessedRecordCount(), greaterThan(0L)));
        stopDatafeed(datafeedConfig.getId());
        assertBusy(() -> assertThat(getJobStats(scrollJobId).get(0).getState(), is(oneOf(JobState.CLOSED, JobState.OPENED))));
        
        if (getJobStats(scrollJobId).get(0).getState().equals(JobState.OPENED)) {
            updateDatafeed(new DatafeedUpdate.Builder().setId(datafeedConfig.getId()).setChunkingConfig(ChunkingConfig.newAuto()).build());
            startDatafeed(
                datafeedConfig.getId(),
                randomLongBetween(0, getDataCounts(scrollJobId).getLatestRecordTimeStamp().getTime()),
                now
            );
            waitUntilJobIsClosed(scrollJobId);
        }
        assertBusy(() -> {
            DataCounts dataCounts = getDataCounts(scrollJobId);
            assertThat(dataCounts.getProcessedRecordCount(), equalTo(numDocs + numDocs2));
            assertThat(dataCounts.getOutOfOrderTimeStampCount(), equalTo(0L));
        }, 60, TimeUnit.SECONDS);