QAN - increase query length and examples/explain

Hi,

as posted in another thread, the examples/explain tabs worked for me after enabling the events_statements_current and events_statements_history consumers.

max_digest_length configures the max length for the normalized version (as shown in the screenshot)

max_sql_text_length configures the max length for the SQL_TEXT column, it is used as example (which is then used for the explain)

https://forums.percona.com/discussion/comment/57538/#Comment_57538