Update protobuf requirement from 2.14 to 3.4

Updates the requirements on [protobuf](https://github.com/stepancheg/rust-protobuf) to permit the latest version.
- [Changelog](https://github.com/stepancheg/rust-protobuf/blob/master/CHANGELOG-before-3.md)
- [Commits](stepancheg/rust-protobuf@v2.14.0...v3.4.0)

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

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

Author: dependabot[bot]

opentelemetry-prometheus/Cargo.toml

@@ -24,7 +24,7 @@ once_cell = { workspace = true }
 opentelemetry = { version = "0.22", path = "../opentelemetry", default-features = false, features = ["metrics"] }
 opentelemetry_sdk = { version = "0.22", path = "../opentelemetry-sdk", default-features = false, features = ["metrics"] }
 prometheus = "0.13"
-protobuf = "2.14"
+protobuf = "3.4"
 
 [dev-dependencies]
 opentelemetry-semantic-conventions = { path = "../opentelemetry-semantic-conventions" }