Add a comment field to a raw events table to allow a user keep arbitrary (helpful) data

[melezhik]

In a spirit of #413

I'd suggest more general thing - to have comment field in raw field table, it would help to debug issues with statistics. For example one could use this field to store information about specific event, for example repository name or application name. In our case we aggregated statistics across many projects , repositories, but I still want to be able to see to which repository , application, pipeline this specific event refers to , this will allow to me access the data in four key dashboard and make a proper judgment about data correctness.