allow projects to provide a custom newrelic.yml

[samhanes-fs]

Describe the Enhancement

Right now the buildpack always packages a default newrelic.yml agent configuration file and there's no way to replace or modify it.

As a developer working on a project which uses this buildpack, I want to provide a custom newrelic.yml file to be used instead of the default.

Possible Solution

The buildpack could look for a newrelic.yml file in a conventional location in the project sources (perhaps at the project root) and use it instead of the default one if it's present.

Motivation

Since there's no way to change newrelic.yml, if you want to change the agent settings you need to either use New Relic Server-Side Configuration or set them one-by-one through environment variables or bindings. The former doesn't support all properties and the latter is cumbersome and doesn't work well for properties with complex values.

For some agent settings, especially the include/exclude lists for errors and logs, it makes more sense for the configuration to live with the application code and be applied equally in all environments, rather than for it to be part of the environment configuration.