Update hashbrown requirement from 0.11 to 0.12 (#266)

Updates the requirements on [hashbrown](https://github.com/rust-lang/hashbrown) to permit the latest version.
- [Release notes](https://github.com/rust-lang/hashbrown/releases)
- [Changelog](https://github.com/rust-lang/hashbrown/blob/master/CHANGELOG.md)
- [Commits](rust-lang/hashbrown@v0.11.0...v0.12.0)

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

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

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

metrics-util/Cargo.toml

@@ -60,7 +60,7 @@ radix_trie = { version = "0.2", default-features = false, optional = true }
 ordered-float = { version = "2.0", default-features = false, optional = true }
 num_cpus = { version = "1", default-features = false, optional = true }
 ahash = { version = "0.7", default-features = false, optional = true }
-hashbrown = { version = "0.11", default-features = false, optional = true, features = ["ahash"] }
+hashbrown = { version = "0.12", default-features = false, optional = true, features = ["ahash"] }
 
 [dev-dependencies]
 approx = "0.5"