Bump github.com/prometheus/client_golang in /exporters/prometheus

Bumps [github.com/prometheus/client_golang](https://github.com/prometheus/client_golang) from 1.11.0 to 1.12.0.
- [Release notes](https://github.com/prometheus/client_golang/releases)
- [Changelog](https://github.com/prometheus/client_golang/blob/main/CHANGELOG.md)
- [Commits](prometheus/client_golang@v1.11.0...v1.12.0)

---
updated-dependencies:
- dependency-name: github.com/prometheus/client_golang
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

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

exporters/prometheus/go.mod

@@ -3,7 +3,7 @@ module go.opentelemetry.io/otel/exporters/prometheus
 go 1.16
 
 require (
-	github.com/prometheus/client_golang v1.11.0
+	github.com/prometheus/client_golang v1.12.0
 	github.com/stretchr/testify v1.7.0
 	go.opentelemetry.io/otel v1.3.0
 	go.opentelemetry.io/otel/metric v0.26.0