[APM] Service overview: Research Dependencies table implementation

[formgeist]

Summary

Related design issue elastic/apm#300

The service overview introduces a new view for the selected service that encompasses a lot of the service data from existing views like metrics, transactions, and errors. The primary goal of the view is to aid in troubleshooting a service from a birds eye view rather than immediately diving into the Transactions, as that's the current landing page for a selected service. There's more information about the user stories and goals of the view in the design issue linked above.

Dependencies table

One of the new components of the view is the dependencies tables that list the downstream services or external connections that are relevant for the service at the selected time range. The information stems from the destination data captured for the service map. In this component, we're not only listing the connections but also showing the metrics per connection such as latency, traffic, and error rate.

We'll need to investigate how we query the connections and their metrics.

Secondly, the listed services can be linked to the service overview page for each of those. Other external connections currently don't have a view e.g. databases, but this is going to come in later additions to the Service views.

[elasticmachine]

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

[formgeist]

Just had a sync with @nehaduggal around the data we will show in this table and we agreed to remove the search health status from the table data because it's confusing to see the service health in relation to show the connection metrics. cc @alex-fedotyev

[dgieselaar]

I'll try to implement this without a scripted_metric aggregation. Created an implementation issue at #81120, so we can close this.