This repo is missing a README

As part of ipfs/docs#55, we’re trying to make sure every repo has a useful readme and description. `go-metrics-prometheus` has a description, but no README at all, so we should make sure we add one.

It doesn’t need to be too special, but we should at least explain:

- Name
- More detailed description that the GitHub description field (even if it’s only one short paragraph). Try and say how it fits into IPFS (e.g. `go-ipfs` relies on this module to send metrics when you configure xyz).
- How to install (even if just `go get xyz` or `git clone xyz`)
- [Standard license text](https://github.com/ipfs/community/blob/master/LICENSING_POLICY.md) — you can pretty much just copy & paste.
- Bonus points for a usage example if it’s easy to whip one up!

Start by reviewing the repository description and the licensing policy linked in the issue. Add a README covering the project name, its relationship to IPFS, installation, and the standard license text; include a usage example if the repository makes one straightforward. Done means the repository has a useful README with the requested sections.

Written by the indexing model from the issue text.