Custom usage counter example and integration test

[filipgolem]

A matching custom runtime:
https://github.com/golemfactory/ya-runtime-sdk/pull/16/files

presets.json:

{
  "ver": "V1",
  "active": [
    "wasm", "custom-counters", "vm"
  ],
  "presets": [
[...]
    {
      "name": "custom-counters",
      "exeunit-name": "custom-counters",
      "pricing-model": "linear",
      "initial-price": 0,
      "usage-coeffs": {
        "golem.usage.duration_sec": 0.0001,
        "golem.usage.cpu_sec": 0.0001,
        "golem.usage.custom.counter": 0.0003
      }
    }
  ]
}

[shadeofblue]

added a few comments - plus, there should be some explanation (in the beginning of te source maybe? or in an accompanying README.md file) what the custom counter is and how it is increased so that the user can understand what's happening ... without some explanation, the whole example is very opaque...

plus, ideally, there should be a comment on how to run the example yourself, without goth

[filipgolem]

I added README.md: https://github.com/golemfactory/yapapi/tree/custom-usage-counter-example/examples/custom-usage-counter

[johny-b]

Is there any reason why we don't do if __name__ == '__main__' here?
Also, what with all this try/except, task.cancel() etc that we have in most examples?

I think this might be quite confusing for the reader who already knows & understands previous examples.

[filipgolem]

Is this necessary if we are not importing this file? It's just an example that is meant to be run directly, it even starts with #!/usr/bin/env python3. I wanted this example to be as simple as possible, handling NoPaymentAccountError and KeyboardInterrupt would make it more complicated. It's not great when most of the code in the example is boilerplate.

[johny-b]

Someone might want to import main from this example and use it in some other way. Why forbid them? It might make "playing with an example" easier.
I think this if __name__ ... is just a good standard, although I won't insist.

About NoPaymentAccountError/KeyboardInterrupt etc: I definitely agree this should be simplified, but I'd rather have the same in all examples (except hello world) and then fix it in all examples at the same time. But I won't insist here either : )

[filipgolem]

I added if __name__ == "__main__": (9b238e6)

BTW: this example does not run indefinitely, so Ctrl+C wouldn't be typically pressed:

while datetime.now() < start_time + timedelta(seconds=self._running_time_sec):
...
parser.add_argument(
    "--running-time",
    default=30,

[shadeofblue]

more or less looks good ...

added two more comments, it would be good @filipgolem if you could still include them

[filipgolem]

more or less looks good ...

added two more comments, it would be good @filipgolem if you could still include them

Done.