chore(deps): update syn requirement from 2 to 3 (#182)

Updates the requirements on [syn](https://github.com/dtolnay/syn) to permit the latest version.
- [Release notes](https://github.com/dtolnay/syn/releases)
- [Commits](dtolnay/syn@2.0.0...3.0.2)

---
updated-dependencies:
- dependency-name: syn
  dependency-version: 3.0.2
  dependency-type: direct:production
...

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Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>
Co-authored-by: Qinxuan Chen <koushiro.cqx@gmail.com>

Author: dependabot[bot]

fastmetrics-derive/Cargo.toml

@@ -21,7 +21,7 @@ proc-macro = true
 [dependencies]
 proc-macro2 = "1"
 quote = "1"
-syn = "2"
+syn = "3"
 
 [dev-dependencies]
 fastmetrics = { path = "../fastmetrics" }