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
assertBusy(() -> assertEquals(getJobStats(job.getId()).get(0).getState(), JobState.OPENED));
        DatafeedConfig.Builder datafeedConfigBuilder = createDatafeedBuilder(
            job.getId() + "-datafeed",
            job.getId(),
            Collections.singletonList("data")
        );
        if (maxEmptySearches != null) {
            datafeedConfigBuilder.setMaxEmptySearches(maxEmptySearches);
        }
        DatafeedConfig datafeedConfig = datafeedConfigBuilder.build();
        putDatafeed(datafeedConfig);
        startDatafeed(datafeedConfig.getId(), 0L, null);
        assertBusy(() -> {
            DataCounts dataCounts = getDataCounts(job.getId());
            assertThat(dataCounts.getProcessedRecordCount(), equalTo(numDocs1));
            assertThat(dataCounts.getOutOfOrderTimeStampCount(), equalTo(0L));
        });
        now = System.currentTimeMillis();
        long numDocs2;
        if (maxEmptySearches == null) {
            numDocs2 = randomIntBetween(2, 64);
            indexDocs(logger, "data", numDocs2, now + 5000, now + 6000);
        } else {
            numDocs2 = 0;
        }
        assertBusy(() -> {
            DataCounts dataCounts = getDataCounts(job.getId());
            assertThat(dataCounts.getProcessedRecordCount(), equalTo(numDocs1 + numDocs2));
            assertThat(dataCounts.getOutOfOrderTimeStampCount(), equalTo(0L));
        }, 30, TimeUnit.SECONDS);