Metrics for when partial aggregation mode is hit

[alamb]

Is your feature request related to a problem or challenge?

@korowa added "skip partial aggregation mode" in #11627 which helps with high cardinality aggregates by doing minimal work for the first phase of the aggregation. This mode is triggered dynamically based on how effective the first aggregation phase is working.

However, it is not clear from the metrics (e.g. the output of EXPLAIN ANALYZE plans) when this mode

Describe the solution you'd like

I would like some sort of metric that can be used to determine when the partial aggregation phase has been invoked

Describe alternatives you've considered

I think the number of convert_to_state rows is a good metric -- 0 means no partial aggregation was done, and more than 1 gives the number of rows that were subject to the partial aggregate.

Additional context

No response

[alamb]

I have a draft PR for this here; #11706