Bump futures from 0.3.14 to 0.3.15 (#3178)

Bumps [futures](https://github.com/rust-lang/futures-rs) from 0.3.14 to 0.3.15.
- [Release notes](https://github.com/rust-lang/futures-rs/releases)
- [Changelog](https://github.com/rust-lang/futures-rs/blob/master/CHANGELOG.md)
- [Commits](rust-lang/futures-rs@0.3.14...0.3.15)

---
updated-dependencies:
- dependency-name: futures
  dependency-type: direct:production
  update-type: version-update:semver-patch
...

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

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

metered-channel/Cargo.toml

@@ -6,10 +6,10 @@ edition = "2018"
 description = "Channels with attached Meters"
 
 [dependencies]
-futures = "0.3.12"
+futures = "0.3.15"
 futures-timer = "3.0.2"
 derive_more = "0.99"
 
 [dev-dependencies]
 assert_matches = "1.4.0"
-futures = { version = "0.3.12", features = ["thread-pool"] }
+futures = { version = "0.3.15", features = ["thread-pool"] }