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
aggs.addAggregator(
                AggregationBuilders.composite(
                    "buckets",
                    Collections.singletonList(
                        new DateHistogramValuesSourceBuilder("timebucket").fixedInterval(new DateHistogramInterval("1h")).field("time")
                    )
                ).subAggregation(AggregationBuilders.max("time").field("time")).size(100)
            );
            updateDatafeed(new DatafeedUpdate.Builder().setId(compositeDatafeedConfig.getId()).setParsedAggregations(aggs).build());
            startDatafeed(
                compositeDatafeedConfig.getId(),
                randomLongBetween(0, getDataCounts(compositeJobId).getLatestRecordTimeStamp().getTime()),
                now
            );
            waitUntilJobIsClosed(compositeJobId);
        }
        List<Bucket> scrollBuckets = getBuckets(scrollJobId);
        List<Bucket> compositeBuckets = getBuckets(compositeJobId);
        assertThat(
            "scroll bucket size " + scrollBuckets + " does not equal composite bucket size" + compositeBuckets,
            compositeBuckets.size(),
            equalTo(scrollBuckets.size())
        );
        for (int i = 0; i < scrollBuckets.size(); i++) {
            Bucket scrollBucket = scrollBuckets.get(i);
            Bucket compositeBucket = compositeBuckets.get(i);
            try {
                assertThat(
                    "scroll buckets " + scrollBuckets + " composite buckets " + compositeBuckets,
                    compositeBucket.getTimestamp(),