pmm-agent moves slow logs without checking privileges

==============Original Ticket======================

On a MySQL Community 5.6 server, slow log rotation is being performed incorrectly.
The agent is using the default setting (server-side config for rotation) and is rotating the slow query log without instructing MySQL server to cleanly handle this.

MySQL details:

$ mysql -Bse "show variables like '%slow%'; show variables like 'long%'; select @@version_comment, @@version"
log_slow_admin_statements       ON
log_slow_slave_statements       ON
slow_launch_time        2
slow_query_log  ON
slow_query_log_file     /var/lib/mysql/xxx-slow.log
long_query_time 0.000000
MySQL Community Server (GPL)    5.6.47-log

Steps to reproduce:

1. Configure pmm-agent to monitor MySQL using slowlog
2. Monitor the slow log file along with the .old path, e.g.
   Example check, log path is configured in full

   slowlog=$(mysql -Bse "select @@slow_query_log_file")
   while [[ ! -f abort-slow-log-check ]]
   do
       ls -la "${slowlog}" "${slowlog}.old" | sed "s/^/$(date --iso-8601=seconds)\t/g"
       sleep 10
   done | tee slow-log-check.log

1. Run sysbench to rapidly generate content for the slow log, e.g.
   Enough to quickly fill default size log

   cat <<EOS > ${HOME}/.sysbench-config
   mysql-socket=/var/lib/mysql/mysql.sock
   mysql-user=REDACTED
   mysql-password=REDACTED
   mysql-db=REDACTED
   threads=5
   tables=5
   scale=5
   EOS
   
   ./tpcc.lua --config-file=${HOME}/.sysbench-config prepare
   

The log will be rotated whilst still being written to.

=========================================================Original Ticket==============================

Bug Reproduced on 2.9.1/dev-latest, Steps to recreate are enough and detailed, reproduced the scenario with Mysql 8.x latest version, we need to update the documentation to mention the needed permission for slow log rotation. 

How to test/QA?
add mysql instance for monitoring with slowlog enabled. In case of a mysql user that doesn't have reload privileges pmm agent will have the following error message in the log saying "RELOAD grant not enabled cannot rotate slowlog", if the mysql user is granted the RELOAD privilege this log message should disappear and slowlog rotation will continue

FB: https://github.com/Percona-Lab/pmm-submodules/pull/1781