Upgrade: bump github.com/prometheus/client_golang from 1.7.1 to 1.8.0

Bumps [github.com/prometheus/client_golang](https://github.com/prometheus/client_golang) from 1.7.1 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](prometheus/client_golang@v1.7.1...v1.8.0)

Signed-off-by: dependabot-preview[bot] <support@dependabot.com>

go.mod

@@ -2,7 +2,7 @@ module github.com/luzilla/dnsbl_exporter
 
 go 1.12
 
-require github.com/prometheus/client_golang v1.7.1
+require github.com/prometheus/client_golang v1.8.0
 
 require (
 	github.com/Luzilla/godnsbl v1.0.0