feat: Log configured Telegraf plugins

__Proposal:__
For customers who generate their Telegraf configs from the cloud it would be very useful for us to get visibility to which Telegraf plugins they are using, so we can prioritize bug fixes/documentation for the more commonly used plugins.

__Desired behavior:__
Some mechanism (log, report etc.) to see which Telegraf plugins are being included in customers' configs, with a count for each plugin type. We don't need any config-specific data.

__Use case:__
Helps to give us visibility to more important plugins.