Update prometheus requirement from 0.12 to 0.13

Updates the requirements on [prometheus](https://github.com/tikv/rust-prometheus) to permit the latest version.
- [Release notes](https://github.com/tikv/rust-prometheus/releases)
- [Changelog](https://github.com/tikv/rust-prometheus/blob/master/CHANGELOG.md)
- [Commits](tikv/rust-prometheus@v0.12.0...v0.13.0)

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

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

Cargo.toml

@@ -45,7 +45,7 @@ metrics = ["opentelemetry/metrics", "opentelemetry-prometheus", "prometheus"]
 opentelemetry = "0.15"
 opentelemetry-prometheus = { version = "0.8", optional = true }
 opentelemetry-semantic-conventions = "0.7"
-prometheus = { version = "0.12", optional = true }
+prometheus = { version = "0.13", optional = true }
 tide = "0.16"
 url = "2.2"
 http-types = "2.11"