[APM] Use new host-specific transaction metrics

[dgieselaar]

APM Server will start generating host-specific transaction metrics (new), and host-agnostic transaction metrics (a change in the current transaction metrics), to combat high cardinality metrics, which leads to memory issues in APM Server.

This means that the UI will need to make the following changes:

• Decide whether host-specific metrics are available and appropriate for the currently selected time range (re-use the same logic as we do today for service metrics)
• Use host-specific transaction metrics where host-specific metadata is requested, e.g. for the instances table, the metrics tab, but also the service (instance) details

See: https://github.com/elastic/apm-dev/issues/1021 (internal)

[elasticmachine]

Pinging @elastic/apm-ui (Team:APM)

[achyutjhunjhunwala]

I would also propose updating Synthtrace to support this change and newly added metrics as part of this scope. As we will also be adding/updating tests

[gbamparop]

We will not be able to complete this in 8.10, we will consider it for 8.11. @elastic/apm-server can we postpone elastic/apm-server#11312 until 8.11?

[gbamparop]

We have to postpone this further and align with @elastic/apm-pm to get it prioritised for another release.