Query Example is often empty

Suggested implementation:

1. Add version check
2. For version mysql 8.0 or higher use events_statements_summary_by_digest.QUERY_SAMPLE_TEXT instead events_statements_history.SQL_TEXT.

How to test:

    1. Add mysql verison 8.0
    2. Add another mysql version lower than 8.0
    3. Check QAN
    4. Check query examples missing on 0% - 40% (average around 25%) by compare result of these two queries:

SELECT count(*) FROM pmm.metrics WHERE service_type = 'mysql'

SELECT count(*) FROM pmm.metrics WHERE (service_type = 'mysql') AND (example = '')

There is often an empty query example in QAN2. Checked in clickhouse, and see that only 23% have query example.

SELECT count(*)
FROM pmm.metrics 
┌──count()─┐
│ 41720863 │
└──────────┘



SELECT count(*)
FROM pmm.metrics 
WHERE example = ''
┌──count()─┐
│ 32305583 │
└──────────┘ 

I do not see any explanation on why example is empty in metrics table, here are related fields:

example:                      
example_format:               EXAMPLE_FORMAT_INVALID
is_truncated:                 0
example_type:                 EXAMPLE_TYPE_INVALID
example_metrics:               

Is there anything else which I could provide to better understand the reason of this? BTW I searched in older issues and found PMM-909 which might be somehow related, I did not reopen it since it's about another version.

 DOD: Source of query example for MySQL 8.0 should be selected from events_statements_summary_by_digest.QUERY_SAMPLE_TEXT