metrics-exporter-prometheus: properly sanitize metric names

[gnuvince]

Closes #289

#289 describes a case when a valid metric name is sanitized (i.e., changed) when it need not be. I added a test to exercise this case and modified the sanitize_metrics_name function to pass this test. In terms of approach, I changed from using methods on strings to using a pre-constructed lookup table. The helper functions that were previously used to implement sanitize_metrics_name have been moved in the test module to act as oracles in the property tests.

[tobz]

@gnuvince Thanks for opening the initial issue and the subsequent PR to fix it.

I think the lookup table approach is a bit too optimized. I know, this might sound silly, but given the pattern of how an exporter is used -- typically on an interval or polled infrequently -- the lookup table-based approach feels like it's trading far too much in terms of obviousness and readability for what is likely a small improvement in overall performance.

I'd like to a see a more iterative, straightforward approach -- replace the initial character, even if it results in a suboptimal clone, then do the remaining steps -- as a first pass of fixing this issue.

[gnuvince]

How about this version? If performance is not much of an issue, I think this is the clearest I can make it.

[tobz]

Looking a lot simpler and straightforward, just one more small tweak.

[tobz]

Released in [email protected].