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
long oneWeekAgo = now - 604800000;
        long twoWeeksAgo = oneWeekAgo - 604800000;
        indexDocs(logger, "data", numDocs, twoWeeksAgo, oneWeekAgo);
        Job.Builder job = createScheduledJob(jobId);
        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.Builder datafeedConfigBuilder = createDatafeedBuilder(datafeedId, jobId, Collections.singletonList("data"));
        
        datafeedConfigBuilder.setChunkingConfig(ChunkingConfig.newManual(new TimeValue(1, TimeUnit.SECONDS)));
        DatafeedConfig datafeedConfig = datafeedConfigBuilder.build();
        putDatafeed(datafeedConfig);
        startDatafeed(datafeedConfig.getId(), 0L, now);
        assertBusy(() -> {
            DataCounts dataCounts = getDataCounts(job.getId());
            assertThat(dataCounts.getProcessedRecordCount(), greaterThan(0L));
        }, 60, TimeUnit.SECONDS);
        try {
            CloseJobAction.Response closeJobResponse = closeJob(jobId, useForce);
            assertTrue(closeJobResponse.isClosed());
        } catch (Exception e) {
            NodesHotThreadsResponse nodesHotThreadsResponse = client().admin().cluster().prepareNodesHotThreads().get();
            int i = 0;
            for (NodeHotThreads nodeHotThreads : nodesHotThreadsResponse.getNodes()) {
                logger.info(i++ + ":\n" + nodeHotThreads.getHotThreads());
            }
            throw e;
        }
        GetDatafeedsStatsAction.Request request = new GetDatafeedsStatsAction.Request(datafeedId);