Instruments Kibana with Elastic APM by adding the Node.js agent to the
source code. The agent is not turned on by default but can be enabled by
setting the environment variable `ELASTIC_APM_ACTIVE=true` or by
creating an apm config file called `config/apm.dev.js` and setting
`active: true` inside of it.

This implementation is not meant to be used by end-users of Kibana as it
lacks integration with the regular Kibana config file. For now, this is
meant as a useful internal tool for Elastic employees when developing
Kibana.

By default, it's pre-configured with a `serverUrl` pointing to an APM
Server hosted on Elastic Cloud. The data is stored in an ES cluster
accessible only by Elastic employees. These defaults can easily be
overwritten using environment variables or via the custom config file.