[Monitoring] Missing apm metric in mapping

Found through elastic/kibana#23958

[This PR](https://github.com/elastic/elasticsearch/pull/33286) introduced the mappings for apm monitoring, but the [UI code](https://github.com/elastic/kibana/blob/master/x-pack/plugins/monitoring/server/lib/metrics/apm/metrics.js#L173) references a field that is not present.

Double checking with @graphaelli this is indeed an issue and needs to be fixed