Reduce unnecessary metric creation in loop

[umax]

Hi!

I will try to improve multiprocess metrics collections by a series of pull requests.
This one fixes unnecessary metric creation in loop.

The more correct way to use dict.setdefault is result = dict.setdefault(key, value). In our case:

metric = metrics.setdefault(metric_name, core.Metric(...))

instead of

metrics.setdefault(metric_name, core.Metric(...))
metric = metrics[metric_name]

But the problem is that core.Metric(...) will be executed each time when setdefault method calls whether needed or not.

[brian-brazil]

Chaining = can be confusing, can you make this two statements?

[umax]

no problem

[brian-brazil]

Thanks!