Counter with histogram suffix will clash with histogram

[pedro-stanaka]

Summary

While running the exporter in production a colegue faced an error when a counter clashes with a previously collected histogram.
I managed to reproduce the bug with some tests cases and this PR fixes the problem.

Bug description

The problem can be reproduced in the following manner.

1. A sample foo:1|h (a histogram - to be translated to a prometheus histogram) is sent to the exporter.
2. The exporter will register foo_count, foo_sum and foo_bucket.
3. Later a sample foo_count:1|c arrives at the exporter, the exporter will also register the counter.
4. The gatherer will detect that this counter is not part of the histogram, but a separate counter and will crash.

Changes

• Fix the described bug, by checking for existing histograms before registering the counters.
• Improve isolation of test case TestConflictingMetrics() by using a new Prometheus registry for each test run.

[matthiasr]

This is great, thank you! Do we need the same check for gauges?

[pedro-stanaka]

This is great, thank you! Do we need the same check for gauges?

Good catch. Added a few more test cases and I am checking now for gauges as well.

[pedro-stanaka]

From what I see, the rest should be covered.

[pedro-stanaka]

Just checking, anything else needed here @matthiasr ?

[matthiasr]

Sorry, my bad, I'm underwater with work. Will review as soon as possible.

[matthiasr]

Thank you!