Is there a way to find out how many times a stored procedure was invoked since last DB start
If you can take the hit on performance, enable tracing for a "typical" session (set tracefile "/some/path/" for some_spid), and using set show_sqltext on you can see all the SQL executed. Then you will need to parse the output looking for stored procedures. Of course this approach cannot give you 100% confidence (e.g. maybe you have a batch job that runs once per year) but it's a start.