Make tablestat messages more descriptive with respect to current status and limits for automatically disabling

$ pmm-admin add mysql --help

usage: pmm-admin add mysql [<flags>] [<name>] [<address>]

[…]
      --disable-tablestats       Disable table statistics collection
      --disable-tablestats-limit=DISABLE-TABLESTATS-LIMIT
                                 Table statistics collection will be disabled if there are more than specified number of tables (default: server-defined)
[…]

MySQL Service added.
Service ID  : /service_id/1
Service name: mysql-1

Table statistics collection enabled (the limit is 1000, the actual table count is 500).

MySQL Service added.
Service ID  : /service_id/1
Service name: mysql-1

Table statistics collection enabled (the table count limit is not set).

MySQL Service added.
Service ID  : /service_id/1
Service name: mysql-1

Table statistics collection enabled (the limit is 1000, the actual table count is unknown).

MySQL Service added.
Service ID  : /service_id/1
Service name: mysql-1

Table statistics collection disabled (the limit is 1000, the actual table count is 2000).

MySQL Service added.
Service ID  : /service_id/1
Service name: mysql-1

Table statistics collection disabled (always).