Import tracing without the attributes feature (#623)

tracing-attributes depends on syn and proc-macro2, which are slow to compile

Author: paolobarbolini

tower/Cargo.toml

@@ -75,7 +75,7 @@ slab = { version = "0.4", optional = true }
 tokio = { version = "1", optional = true, features = ["sync"] }
 tokio-stream = { version = "0.1.0", optional = true }
 tokio-util = { version = "0.6.3", default-features = false, optional = true }
-tracing = { version = "0.1.2", optional = true }
+tracing = { version = "0.1.2", default-features = false, features = ["std"], optional = true }
 pin-project-lite = "0.2.7"
 
 [dev-dependencies]