Improve cyclomatic complexity

[ilya-anashkin]

Hi! Using the example of my PR, I would like to show that the library needs to improve its cyclomatic complexity.
I tested with the utility radon

My changes

SummaryMetricFamily.__init__ - B (7)
to
SummaryMetricFamily.__init__ - B (6)

HistogramMetricFamily - B(7)
to
Deleted from >=B level complexity

HistogramMetricFamily.__init__ - B(7)
to
HistogramMetricFamily.__init__ - B(6)

CollectorRegistry._get_names - B(7)
to
CollectorRegistry._get_names - B(6)
```

```
629 blocks (classes, functions, methods) analyzed.
Average complexity: A (2.1701112877583464)

to

629 blocks (classes, functions, methods) analyzed.
Average complexity: A (2.1701112877583464)
```

[csmarchbanks]

Thank you for your contribution, I am not sure if this is an improvement over the old code, generally lowering cyclomatic complexity isn't a goal in and of itself, the goal is the most readable code. I think some of these changes help with readability, e.g. the change to registry.py, but not all of them.

[csmarchbanks]

I find the old logic to be clearer. It was clearly a check at the very beginning to make sure the input was valid, then we assigned a default value. Now we are doing both at the same time which IMO is harder to understand.