Document database metric batch average operation

What does this MR do and why?

This merge request adds the documentation for the changes introduced in !89913 (merged). It explains how to use the new average operation and shows different examples similar to other database metrics operations.

Resolves #364815 (closed).

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.