Update prometheus-client to 0.6.0

[pyup-bot]

This PR updates prometheus-client from 0.5.0 to 0.6.0.

Changelog

0.6.0

[ENHANCEMENT] Better exceptions on exposition failure (364)
[BUGFIX] Fix deadlock in gcCollector, metrics are now different (371)
[BUGFIX] Fix thread leak in Python 3.7 (356)
[BUGFIX] Make the format strings compatible with Python 2.6 (361)
[BUGFIX] parser: ensure samples are of type Sample (358)

Links

• PyPI: https://pypi.org/project/prometheus-client
• Changelog: https://pyup.io/changelogs/prometheus-client/
• Repo: https://github.com/prometheus/client_python

[pyup-bot]

Closing this in favor of #473