Add agent api example with prometheus and grafana

[dhurley]

Proposed changes

• Added an example of how to setup a Grafana dashboard with the NGINX agent.
• Updated the code to not add the file watcher plugins when no GRPC server is configured in the agent configuration.
• Updated registration plugin to read the NGINX config when registering an agent so that things like access/error logs are updated on the startup of the agent when no GRPC server is configured.

Checklist

Before creating a PR, run through this checklist and mark each as complete.

• I have read the CONTRIBUTING document
• If applicable, I have added tests that prove my fix is effective or that my feature works
• If applicable, I have checked that any relevant tests pass after adding my changes
• I have updated any relevant documentation (README.md)