Add v2 write + query endpoints

[danquack]

I ran into a use case where I needed a v2 client to write to the exporter. Although there might be some endpoints of interest, simply pointing my v2 client at http://localhost:9122 allowed for the metrics to write. One thing to note is this ignores all the token, org, and buckets feature set provided with v2.

Its worth noting the contributing doc didn't specify updating the version file.

Relates #80

@matthiasr

[matthiasr]

This is great! Could you add a note to the README about v2 support and document the limitations there?

[matthiasr]

Its worth noting the contributing doc didn't specify updating the version file

That's right, I do that when I cut a release. Generally multiple PRs go into one release, in different order than they were submitted; it's only in this relatively quiet repository that I cut a release after every significant change.

[matthiasr]

Thank you!