[APM] Backends UI: Backend landing page

[formgeist]

Summary

Meta issue: #103098

The backend page will feature an overall performance health view of the backend based on the metrics from all callees. The list of upstream services breaks down the health of the callee services.

The operations breakdown and analysis is not in scope for 7.15 as we're dependent on changes in the agents and server to get span metrics.

Solution

Metrics panels

The three metrics panels at the top are derived from the destination metrics aggregated for the selected backend. Only average aggregations available for latency for these metrics as they don't support histograms.

Upstream services list

The list of upstream services will list the immediate 1-step list of callees to the selected backend. For each we will show the actual service health and performance metrics.

[elasticmachine]

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