Add VictoriaMetrics support from sql to promsql about octosql

[cube2222]

I understand Prometheus semantics and I've used it a lot. The naive, fairly ergonomic implementation would be very easy to do even in a performant way. However, Prometheus seems to be more of a streaming data source in the context of octosql, so in order to achieve a coherent system, we'll have to hold off with Prometheus support till we have streaming semantics (temporal sql extensions) and decide if they fit Prometheus or not.

Feel free to implement Prometheus with the timestamp as a filterable column, but keep in mind that we may rewrite it with a different sql interface.

[cube2222]

Prometheus would be fairly easy to add (column per label, with filtering on those and on timestamps).

However, prometheus has semantics which may better fit a streaming context. Which is why this will have to wait until ergonomic streaming support is designed.

[cube2222]

Created an issue to track it. #127

[valyala]

@cube2222 , the following article could be helpful in understanding PromQL semantics - https://medium.com/@valyala/promql-tutorial-for-beginners-9ab455142085