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(response.getResponse().results().get(0).getDatafeedState(), equalTo(DatafeedState.STOPPED));
        });
    }
    /**
     * Stopping a lookback closes the associated job _after_ the stop call returns.
     * This test ensures that a kill request submitted during this close doesn't
     * put the job into the "failed" state.
     */
    public void testStopLookbackFollowedByProcessKill() throws Exception {
        client().admin().indices().prepareCreate("data").setMapping("time", "type=date").get();
        long numDocs = randomIntBetween(1024, 2048);
        long now = System.currentTimeMillis();
        long oneWeekAgo = now - 604800000;
        long twoWeeksAgo = oneWeekAgo - 604800000;
        indexDocs(logger, "data", numDocs, twoWeeksAgo, oneWeekAgo);
        Job.Builder job = createScheduledJob("lookback-job-stopped-then-killed");
        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));
        List<String> t = Collections.singletonList("data");
        DatafeedConfig.Builder datafeedConfigBuilder = createDatafeedBuilder(job.getId() + "-datafeed", job.getId(), t);
        
        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));