Update prometheus requirement from 0.13 to 0.14

Updates the requirements on [prometheus](https://github.com/tikv/rust-prometheus) to permit the latest version.
- [Changelog](https://github.com/tikv/rust-prometheus/blob/master/CHANGELOG.md)
- [Commits](tikv/rust-prometheus@v0.13.0...v0.14.0)

---
updated-dependencies:
- dependency-name: prometheus
  dependency-type: direct:production
...

Signed-off-by: dependabot[bot] <[email protected]>

Author: dependabot[bot]

Cargo.toml

@@ -12,7 +12,7 @@ keywords = ["rocket", "prometheus", "instrumentation", "monitoring"]
 readme = "README.md"
 
 [dependencies]
-prometheus = { version = "0.13", default-features = false }
+prometheus = { version = "0.14", default-features = false }
 rocket = "0.5.0"
 
 [dev-dependencies]