Update documentation about MySQL metrics.
Update documentation about MySQL metrics.

Add the description of two handlers (delete and external locks).

Change-Id: I513f9f412a228894b87066f661a88d3b7edcdd3e

stackforge/fuel-plugin-lma-collector,stackforge/fuel-plugin-lma-collector,stackforge/fuel-plugin-lma-collector,stackforge/fuel-plugin-lma-collector,stackforge/fuel-plugin-lma-collector