build(deps): update tokio-metrics requirement from 0.3 to 0.4

Updates the requirements on [tokio-metrics](https://github.com/tokio-rs/tokio-metrics) to permit the latest version.
- [Release notes](https://github.com/tokio-rs/tokio-metrics/releases)
- [Changelog](https://github.com/tokio-rs/tokio-metrics/blob/main/CHANGELOG.md)
- [Commits](tokio-rs/tokio-metrics@v0.3.1...v0.4.0)

---
updated-dependencies:
- dependency-name: tokio-metrics
  dependency-type: direct:production
...

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

kubert-prometheus-tokio/Cargo.toml

@@ -18,5 +18,5 @@ unexpected_cfgs = { level = "warn", check-cfg = ['cfg(tokio_unstable)'] }
 [dependencies]
 prometheus-client = "0.22"
 tokio = "1"
-tokio-metrics = "0.3"
+tokio-metrics = "0.4"
 tracing = "0.1"