[Lens] Replace scripted metric with scripted terms aggregation

[wylieconlon]

In the Lens telemetry we are using a slow-running scripted metric query: https:/elastic/kibana/blob/master/x-pack/plugins/lens/server/usage/visualization_counts.ts#L10

This query can be replaced by a terms aggregation with script, where the script is used to calculate just the term. I believe the entire script we want is:

String visType = doc['lens.visualizationType'].value;
String niceType = visType == 'lnsXY' ? doc['lens.state.visualization.preferredSeriesType'].value : visType;
state.types.put(niceType, state.types.containsKey(niceType) ? state.types.get(niceType) + 1 : 1);

[elasticmachine]

Pinging @elastic/kibana-app (Team:KibanaApp)

[timroes]

I'd currently hold of on optimizations about that, since given #61663 (comment) it might mean that we in the future anyway need to change the way we collect that telemetry data and can only do it Kibana side in memory.

[wylieconlon]

@timroes I don't see why we would hold off on this optimization: it feels like exactly the kind of small tech debt item that we should do during a fix it week, and it seems to have no impact on the larger discussion that you're having.

[wylieconlon]

Removing this use of the scripted metric in Kibana would assist the Elasticsearch team in some of their cleanup efforts, they've identified that this is one of the few places in the stack that uses this aggregation.