Configuration loader for the APM instrumentation script.
This module is only meant to be used by the APM instrumentation script (src/apm.js)
to load the required configuration options from the kibana.yaml configuration file with
default values.
@kbn/config is the recommended way to load and read the kibana configuration file,
however in the specific case of APM, we want to only need the minimal dependencies
before loading elastic-apm-node to avoid losing instrumentation on the already loaded modules.