Refactor query generation logic into its own crate

[emschwartz]

@hatchan suggested refactoring out the logic for building the Prometheus queries into a separate crate so it can potentially be used outside of the macro.

Maybe we call it something like autometrics-query or autometrics-queries?

[flenter]

Could the library use templates that are perhaps stored in the autometrics-shared repo? That way many more libraries can re-use the logic

[emschwartz]

Good idea @flenter!

I think the best way to do this would be to write the queries as mustache templates and then use something like git subtree to include them in the other repos where we need them.