[Observability / APM] Improve/clarify what is getting queried in the APM Application Kibana application

APM Server/Integration (and MIS) produce aggregated metrics per service.

Still, this is not really obvious to the user, especially because the Kibana Applications app to visualize Services & traces do not use the raw transactions anymore but rolled-up/pre-aggregated metrics of the transactions/spans.

Those rolled-up/pre-aggregated metrics are documented at https://www.elastic.co/docs/solutions/observability/apm/metrics but it's very opaque to the user that, for example, they cannot anymore filter their services by some label they've attached to the context/transactions of their services for RUM application as the aggregated metrics strip out the labels (https://github.com/elastic/apm-server/issues/10532) since 8.7

We should make it more prominent in the docs (which could be the RUM JS docs or the APM O11y docs or both) as a very low % of users would know that they're not looking at raw events when navgating the page.