Errors out when scraping CreateDate

[wuarthur]

when I'm trying to export CreateDate the exporter fails because of an error in exposition.py in prometheus/client_python

More specifically when i have this line in my metrics.yaml
Users[].{create_date: CreateDate, value: 1}

The error is in this function, when v is expected to be a string, however CreateDate is returned as an datetime object, which cause v.replace('\\', r'\\') errors out.

this can be fixed by changing line 78 in exposition.py to k, str(v).replace('\\', r'\\').replace('\n', r'\n').replace('"', r'\"'))

I have made a fork here with the changes