[APM] Add inline help documentation for updating the default `service.name`

[formgeist]

Pending default naming schema change

• https://github.com/elastic/apm/issues/571

Summary

This is related to https://github.com/elastic/apm/issues/382 where the agents are supporting zero-config instrumentation when e.g. using the auto-attachment options. By default the agent will assign a sensible name based on the application information, but if one cannot be found it will use a default naming schema for assigning a name to the service.

We've asked the agents team to align the default naming schema when assigning a default name when a sensible service.name is not available. The alignment effort can be tracked in https://github.com/elastic/apm/pull/556

When using the default service.name (e.g. unknown-java-service), we want to ensure the user is made aware that the name can be changed by using the available environment variable which can be run when starting up the application. That's when we choose to display the icon and popover in the page header.

Design solution

We want to add a small inline help documentation icon and popover in the service page header that will display the general instructions for updating the service.name using the environment variable. The popover will also feature a per-agent relevant docs link describing in more detail the environment variables.

Based on the other inline docs implementations in Kibana, we can provide a popover with in-app documentation relevant to the selected agent type.

[elasticmachine]

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