[Infra UI] Support custom fields in Metric-dropdown

[makwarth]

Motivation

Enable customers to customize the active Metric in the Infra UI. We already implemented this for the Group By dropdown in 6.6

Proposal

For both Metric and Group by, add/move dropdown for searching and selecting a custom field to the top of the dropdown element. Mockup:

For Metric, we'd ideally only show fields that contains metrics. For Group By, only keywords.

Selecting a field in the auto-suggestion popover, will pin that field to the dropdown and activate it. (Like it works for Group By now). Custom fields are currently "just" stored as part of the URL.

UI polish

It'd be nice if the auto-suggestion popover had the width of its content, so users don't end up with this and need to hover each suggestion to see complete field name.

[elasticmachine]

Pinging @elastic/infrastructure-ui

[simianhacker]

A couple of challenges stand out to me....

Just picking a field is not enough because we need to know "how" to aggregate the metric. A good example is anything that's a "counter" style metric like system.network.bytes.out requires derivative(max(system.network.bytes.out)) to display the rate.

There are also metrics like disk usage (and network usage), we don't have a single metric, the user will need to specify which disk (or interface) they are concerned with. They might need to do something like avg(system.filesystem.used.pct{system.filesystem.mount_point:/data}).

[makwarth]

@simianhacker Yeah, you're right. As for the metrics like disk and network usage, the group by dropdown will help, or..? For aggs, does it make sense to have a few pre-build aggregation options, like counter to rate? The user will manually have to select and agg (unless we do some guessing on the metric name) and will only help in common cases. Thoughts?

[ruflin]

Having a manual option to get started sounds like a good idea. Long term I would like to see that Beats tells Kibana / the UI what kind of metric it actually is (counter, gauge) so the UI can automatically make this decision.

[elasticmachine]

Pinging @elastic/logs-metrics-ui (Team:logs-metrics-ui)

[simianhacker]

Closing in favor of #50193