Bump rand from 0.6.3 to 0.6.4

Bumps [rand](https://github.com/rust-random/rand) from 0.6.3 to 0.6.4.
- [Release notes](https://github.com/rust-random/rand/releases)
- [Changelog](https://github.com/rust-random/rand/blob/master/CHANGELOG.md)
- [Commits](https://github.com/rust-random/rand/compare/0.6.3...0.6.4)

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

metrics/Cargo.toml

@@ -17,7 +17,7 @@ sys-info = "0.5.6"
 solana-sdk = { path = "../sdk", version = "0.12.0" }
 
 [dev-dependencies]
-rand = "0.6.3"
+rand = "0.6.4"
 
 [lib]
 name = "solana_metrics"