Allow DESCRIBE queries on read-only connections

[dylanahsmith]

@rafaelfranca

Problem

We want to allow employees to query for production table metadata by allowing them to make raw queries on a restricted connection. One of the statements they have been using is DESCRIBE or DESC which are aliases for EXPLAIN. However, when the mysql connection configuration uses replica: true then this results in a ActiveRecord::ReadOnlyError

Solution

I've added :describe and :desc to the read-only query whitelist for mysql.

[georgeclaghorn]

Backported to 6-0-stable in d7073d3.