HOLI-10700: add anonymous stats API with counter

Adds a new GraphQL API endpoint `anon_count` that takes a counter name as argument. It will insert a row into a counter model for every call with counter name and timestamp as data. To evaluate the counters value, a query needs to count the number of rows for the whole or a specific time frame. Counter evaluation is out-of-scope and left to be done manually at this point in time.