Account metrics

[Victor-N-Suadicani]

Purpose

Implements the account metrics query.

Changes

The implementation is very similar to the transaction metrics query. I needed to add an index to ensure efficiency. I have not thoroughly tested the query but I imagine we can do this later.

Checklist

• My code follows the style of this project.
• The code compiles without warnings.
• I have performed a self-review of the changes.
• I have documented my code, in particular the intent of the
  hard-to-understand areas.
• (If necessary) I have updated the CHANGELOG.