[esaggs] Bucket aggregation without a metric should be supported

[wylieconlon]

Part of #61768

I would expect that an implicit Count metric would be inserted by esaggs when not configured by the user. This would solve the issues we are seeing in Lens, which are:

• Users should be allowed to create a table without metrics [Lens] Datatable without metric column shows no results #64416
• The Terms aggregation should default to sorting by count, even if the user doesn't want to see the count

[elasticmachine]

Pinging @elastic/kibana-app-arch (Team:AppArch)

[flash1293]

Looked into this with @dej611 and discovered a confusing behavior around that at the moment:
If there are no metrics configured, esaggs just returns an empty table as long as metricsAtAllLevels are set to false, but it crashes with an uncaught exception if it's set to true. This will probably be fixed once we support metric-less esaggs calls explicitly.