Fix Prometheus scraping for Python 3

[ofek]

Motivation

        for line in fd:                                                               
            line = line.strip()                                                       
                                                                                      
>           if line.startswith('#'):                                                  
E           TypeError: startswith first arg must be bytes or a tuple of bytes, not str
                                                                                      
.tox\py36-cockroachdb\lib\site-packages\prometheus_client\parser.py:138: TypeError    

Additional Notes

We do the same thing here https://github.com/DataDog/integrations-core/pull/2117/files#diff-4e5d5c986dbd1ca04f508a0cd7f1c8e5R40

[codecov-io]

Codecov Report

Merging #2128 into master will decrease coverage by 13.73%.
The diff coverage is 100%.