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/src/main/java/org/elasticsearch/xpack/ml/datafeed/extractor/scroll/ScrollDataExtractorFactory.java
private final Client client;
    private final DatafeedConfig datafeedConfig;
    private final Job job;
    private final TimeBasedExtractedFields extractedFields;
    private final NamedXContentRegistry xContentRegistry;
    private final DatafeedTimingStatsReporter timingStatsReporter;
    private ScrollDataExtractorFactory(
        Client client,
        DatafeedConfig datafeedConfig,
        Job job,
        TimeBasedExtractedFields extractedFields,
        NamedXContentRegistry xContentRegistry,
        DatafeedTimingStatsReporter timingStatsReporter
    ) {
        this.client = Objects.requireNonNull(client);
        this.datafeedConfig = Objects.requireNonNull(datafeedConfig);