Documentation for CubeStore metrics in app_metrics.rs

Hi Cube team 👋,

I’ve been working on integrating CubeStore metrics with Prometheus via StatsD and came across the metric definitions in [rust/cubestore/cubestore/src/app_metrics.rs](https://github.com/cube-js/cube/blob/master/rust/cubestore/cubestore/src/app_metrics.rs).

While I can see all the metrics (e.g., cs.sql.query.data, cs.streaming.rows, cs.metastore.write_operation.ms, etc.), I couldn’t find any official documentation or descriptions explaining what each of them represents.

Is there any existing document or reference that explains what these metrics represent?

Thanks a lot !

Start with rust/cubestore/cubestore/src/app_metrics.rs and review the metric definitions, including cs.sql.query.data, cs.streaming.rows, and cs.metastore.write_operation.ms. Document what each CubeStore metric represents in an official reference, making the result easy to use with Prometheus via StatsD.

Written by the indexing model from the issue text.