add metric counters for async operations

[butonic]

• [ ] For the dashboard we are interested in the number of postprocessing steps
• [ ] For tests we are interested in
  • the number of shares that still need to be accepted (the async share accepting goroutines in the frontend) as well as
  • the number of postprocessing steps above

cc @individual-it @micbar anything else?

[individual-it]

Added the task to the QA board, so that we don't forget to use that feature in the tests