In your opinion, what are the main KPI`s (key performance indicators) for bar/pub/cafe?

A really important one is footfall - the number of people who actually come in.  You could look at how long they stay, how much they spend, and the proportion who return.

You could also, for example, look at the profitability of small groups versus large groups and arrange your tables accordingly.   Or, at a bar, waiting time to be served.