WIP: Dimensional metric reporting

[tpitale]

• Adds NewRelic.report_dimensional_metric/4

[CLAassistant]

Thank you for your submission! We really appreciate it. Like many open source projects, we ask that you sign our Contributor License Agreement before we can accept your contribution.

---

Tony Pitale seems not to be a GitHub user. You need a GitHub account to be able to sign the CLA. If you have already a GitHub account, please add the email address used for this commit to your account.
_{You have signed the CLA already but the status is still pending? Let us recheck it.}

[tpitale]

Closing in favor of: https://github.com/newrelic/elixir_agent/pull/408