Bump github.com/prometheus/client_golang from 1.1.0 to 1.8.0

Bumps [github.com/prometheus/client_golang](https://github.com/prometheus/client_golang) from 1.1.0 to 1.8.0.
- [Release notes](https://github.com/prometheus/client_golang/releases)
- [Changelog](https://github.com/prometheus/client_golang/blob/master/CHANGELOG.md)
- [Commits](https://github.com/prometheus/client_golang/compare/v1.1.0...v1.8.0

)

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