Add RollupAction support for AggregateDoubleMetric fields

[talevy]

this commit adds support for aggregate_metric_double fields
in the RollupShardIndexer.

Previously, rollups of rollups were failing because the indexer
did not know how to fetch field values for aggregate metric fields and
the respective _doc_count fields

[elasticmachine]

Pinging @elastic/es-analytics-geo (Team:Analytics)

[talevy]

More validation is likely necessary for rollups on agg-metric fields that fail, for example:

• invalid interval is configured for a specific metric. This scenario will need to leverage the mapping meta fields for this information and resolution
• invalid metrics. For example doing a max on a field with only an avg value.

[talevy]

closing due to staleness 👋