I think it'd be very valuable to be able to request and monitor all groups when fetching the node statistics: /_nodes/_local/stats?groups=_all.

I don't have an overview what metrics we would add compared to the existing stats collector. If it makes sense (as @nomoa indicates with the <3), we can discuss the technical implications (compatibility with the existing stats collector, json unmarshaling, ...). I'm a bit conservative with adding more and more collectors to the exporter that increase the number of http calls and load on the ES nodes, but if provides a significant benefit to the majority of users, we can of cause discuss it.

Can you please elaborate a bit on it? I currently don't have much time to work on this, but I'm happy to review PRs

This feature would not require another api call I think as it would fit in the current /_nodes[/_local]/stats call I think, perhaps just a new param to the collector (e.g. -es.stat_groups?).

Stats groups allow application developers to tag their elasticsearch queries with a specific group, elasticsearch will then make these groups available through the stats API.
These groups are usually meant to distinguish different kind of searches whose performance varies:

• a simple fast autocomplete search
• a costly phrase search
• anything that the application developer might think is interesting to track perf for

By scanning the groups the exporter could generate the same elasticsearch_indices_search_* metrics with a new stat_group label.

This would allow to graph a specific group (e.g. fetch times of autocomplete queries vs fulltext search queries).

I fully agree with @onimisionipe and @nomoa, this is a feature that would allow to compare different dashboard performance, or compare different queries performance based on their target index. It would allow to benchmark specific queries and understand optimizations once they are in place.