Use rayon to speed up subscribe message serializing

backend/telemetry_core/Cargo.toml

@@ -20,6 +20,7 @@ num_cpus = "1.13.0"
 once_cell = "1.8.0"
 parking_lot = "0.11.1"
 primitive-types = { version = "0.9.0", features = ["serde"] }
+rayon = "1.5.1"
 reqwest = { version = "0.11.4", features = ["json"] }
 rustc-hash = "1.1.0"
 serde = { version = "1.0.126", features = ["derive"] }
@@ -39,4 +40,4 @@ criterion = { version = "0.3.4", features = ["async", "async_tokio"] }
 
 [[bench]]
 name = "subscribe"
-harness = false
+harness = false