Update prometheus requirement from 0.7 to 0.8

Updates the requirements on [prometheus](https://github.com/pingcap/rust-prometheus) to permit the latest version.
- [Release notes](https://github.com/pingcap/rust-prometheus/releases)
- [Changelog](https://github.com/tikv/rust-prometheus/blob/master/CHANGELOG.md)
- [Commits](https://github.com/pingcap/rust-prometheus/compare/v0.7.0...v0.8.0)

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

Cargo.toml

@@ -41,7 +41,7 @@ toml = "0.5"
 
 [dependencies.prometheus]
 optional = true
-version = "0.7"
+version = "0.8"
 default_features = false
 features = ["process"]