Update env_logger requirement from 0.7 to 0.8 (#108)

Updates the requirements on [env_logger](https://github.com/env-logger-rs/env_logger) to permit the latest version.
- [Release notes](https://github.com/env-logger-rs/env_logger/releases)
- [Changelog](https://github.com/env-logger-rs/env_logger/blob/master/CHANGELOG.md)
- [Commits](https://github.com/env-logger-rs/env_logger/compare/v0.7.0...v0.8.1)

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Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

metrics-benchmark/Cargo.toml

@@ -6,7 +6,7 @@ edition = "2018"
 
 [dependencies]
 log = "0.4"
-env_logger = "0.7"
+env_logger = "0.8"
 getopts = "0.2"
 hdrhistogram = "7.0"
 quanta = "0.6"