Add CF Metrics Exporter to Java build pack

[stokpop]

We have created a cf-metrics-exporter agent that submits custom metrics (requests per seconds from Spring Boot apps) to the cf custom metrics endpoint so it can be used in cf auto scaler.

The repository is here: https://github.com/rabobank/cf-metrics-exporter

The agent jar is released in Github and in Maven Central.

Can this jar be added to the java build pack? We have a clone of the repository with which we can already create an custom offline build pack that includes the cf-metrics-exporter. So we can provide a PR.