I remember sparks good inch in length from the end of a tower crane hook in high wind, we checked all earthed and it was, and I would not want to touch the hook or even put a meter probe on it, nylon strops stopped the banksman getting shocks, but clearly one simply did not touch the hook in high wind, it was at time tallest tower crane in UK.
However in the home we don't have high winds, static can be generated by some thing rubbing, like a fan belt on a car, but in the home one of two options, everything bonded, or RCD protection or of course both, so if RCD protection any large earth fault should trip the RCD, and if bonding is used, then sink should be bonded, so fault from else where, and you have become charged, and the sink has earthed you, but microwave and sink both should be bonded, and so that shock means there is some thing wrong.
The idea of using your cap to turn a light switch on/off because you know you get shocks from it is flawed, today we locate the fault and remove it, it does need testing as next time you may not be so lucky.
If you have RCD protection I would not worry too much, if not static then the RCD would trip, I would press the test button to make sure it works, but would not worry too much, if no RCD protection I would get an electrician to test, will not take long, so should not cost much, but could save your life.