Protect against possible problems with EXPLAIN of stored functions in MySQL

See  https://bugs.mysql.com/bug.php?id=67632 

The problem: MySQL has a strange way of executing EXPLAIN. This might be dangerous for people to execute if they have some stored functions in DB.

See also: blog post on percona.com named  "Uncommon Sense MySQL - When EXPLAIN can trash your database"  (not posted as of now )

Proposed solution:

• use  BEGIN; ROLLBACK for EXPLAIN commands PMM run against DB 
  • It can be manual Explain executed from UI (QAN) against the query 
  • pmm-agent executes EXPLAIN to get the list of the tables included in the table 

Acceptance Criteria

• PMM running Explain commands  and collecting metrics for "harmful" query - doing no farm to the system 

How to test

• Log in into the Mysql database monitored by PMM
• Create database, table and stored function according to https://www.percona.com/blog/2020/11/23/uncommon-sense-mysql-when-explain-can-trash-your-database/
• Run query `select * from (select cleanup()) as t1clean;` at least once so it gets shown in PMM Query Analysis Dashboard. Fill the table with data again.
• Navigate to the QAN dashboard, find and click at the query `select * from (select cleanup()) as t1clean;` and navigate to Explain tab.
• Verify that data in the table did not get deleted.