PMM Add Remote Instance: User can specify 'Table Statistics Limit' for MySQL and AWS RDS MySQL to disable table stat metrics which can have an adverse impact on performance with too many tables

The problem: 

When the user is adding Remote Mysql or  RDS Mysql it's not able to specify Table stat limit  ( https://www.percona.com/doc/percona-monitoring-and-management/2.x/manage/conf-mysql-disable-table-stats.html ) This can lead to a significant overload of performance of DB during the metrics collections 

Proposed solution:

Let the user set the value for disable-tablestat during adding remote Mysql or RDS Mysql instances 

UI: 

Add two UI elements

• a toggle button with three states
  • Disabled (value: -1)
  • Default (value: 1000)
  • Custom: a valid positive integer
• an additional NumberInputField, which behaves as follows:
  • is disabled when the toggle is in `Disabled` state, takes the value of -1
  • is disabled when the toggle is in `Default` state, takes the value of 1000
  • is enabled when the toggle is in `Custom` state, takes the value from the user, but disallows the user to put negative values

Table stat limit [  1000  ]  

description/tooltip:

"disables table statistics collection when there are more than the specified number of tables in the instance".

Possible values:

0 (means a default value of 1000) - BE will disable the metrics if the number of tables exceeds 1000

any positive value - BE will disable the metrics if the number of tables exceeds that value

negative value - to completely disable tablestat & relevant collectors

read more - https://www.percona.com/doc/percona-monitoring-and-management/2.x/manage/conf-mysql-disable-table-stats.html "

How to test:

• specify a negative value and see if the stats are disabled and verify that tablestat metrics & perf_schema.file_instances are not present
• create a database with 10 tables and set value between 1 and 10; then check the same as above
• create a database with 10 tables and set value equal to 1000; then check that the metrics are present
• create a database with 10 tables and set value equal to 0; then check that the metrics are present
• create a database with 1001 tables and set value equal to 0; then check that the metrics are not present