Metrics SDK: implement Prometheus exporter

[bio-aeon]

Resolves #751

A number of things to pay attention to:

1. Exporter is implemented in the separate module sdk-exporter-prometheus.
2. This PR doesn't implement Prometheus "created timestamps" feature, while OpenTelemetry Java does through using of Prometheus client.
3. Maybe we need to discuss, what should config option look like, which allows to add resource attributes as metric attributes. Currently this PR doesn't contain this option. Or maybe we can implement it in a separate PR, since it's not mandatory according to the spec.
4. The spec says that monotonic Sum with delta temporality should be converted to cumulative temporality, but I don't see any OpenTelemetry SDKs doing such a conversion for Prometheus exporter, they usually just exclude everything with delta temporality, which is also done in this PR.
5. Instead of using ScalaCheck generators, this PR uses static "expected" values, because for me it looks like exactly for these tests the complexity of making generated samples ​​isn't much less than writing the exporter itself :) Test cases are inspired by opentelemetry-rust.

[iRevive]

I will take a look over the weekend.

[iRevive]

Maybe we need to discuss, what should config option look like, which allows to add resource attributes as metric attributes. Currently this PR doesn't contain this option. Or maybe we can implement it in a separate PR, since it's not mandatory according to the spec.

I would keep it for the separate PR.

The spec says that monotonic Sum with delta temporality should be converted to cumulative temporality, but I don't see any OpenTelemetry SDKs doing such a conversion for Prometheus exporter, they usually just exclude everything with delta temporality, which is also done in this PR.

I think it's good enough.

[iRevive]

Excellent work.

We can move forward once we decide what to do with the default parameters.

[lenguyenthanh]

this is excellent, can't wait to try it, thanks a lot for the work!

[bio-aeon]

Thanks for the review and feedback. I'll deal with comments in the next few days.

[bio-aeon]

@iRevive I made changes according to the comments, and I also realized that ideally attributes for scope/target info should also go through full preparation, escaping and so on, rather than just call .toString. So, finalized this too.

[iRevive]

Looks good, thanks!

[lenguyenthanh]

@iRevive could we have a release (snapshot is okay) for this? I'd like to try.

[iRevive]

@lenguyenthanh definitely. The snapshot should be available today