[Monitoring] Missing apm metric in mapping

[chrisronline]

Found through #23958

This PR introduced the mappings for apm monitoring, but the UI code references a field that is not present.

Double checking with @graphaelli this is indeed an issue and needs to be fixed

[elasticmachine]

Pinging @elastic/apm-ui

[graphaelli]

Great find, that is indeed reported by apm-server but not in the mapping. Span also reports two others at the moment - frames & stacktraces - but they are not used by the UI and not generally useful.

In addition, we are also reporting a new event type from libbeat which is not in the mapping yet, and not in the UIs - beats_stats.metrics.libbeat.output.events.toomany - I can open an issue to handle that separately.

[chrisronline]

@graphaelli Awesome, good to know. I've added both fields in the linked PR (as well as others I'm finding are missing too). The testing we're adding through #23958 should prevent this from happening in the future