Don't trace tasks spawned through the console subscriber by default

Enabling the console subscriber starts a server that generates a lot of
async activity.  Introspecting or logging that can drown out the
activity the program has outside of instrumentation.

Provide a new builder method, enable_self_trace, to control whether to
also trace events that come with enabling the tokio console.

If self trace is not enabled, set a thread-local subscriber that drops
events coming from the server thread.

Raise the requirement on tracing-core so that setting a per-thread
NoSubscriber is effective.

Author: g2p

console-subscriber/Cargo.toml

@@ -36,7 +36,7 @@ tokio-stream = "0.1"
 thread_local = "1.1.3"
 console-api = { version = "0.2.0", path = "../console-api", features = ["transport"] }
 tonic = { version = "0.7", features = ["transport"] }
-tracing-core = "0.1.18"
+tracing-core = "0.1.24"
 tracing = "0.1.26"
 tracing-subscriber = { version = "0.3.0", default-features = false, features = ["fmt", "registry"] }
 futures = { version = "0.3", default-features = false }
@@ -58,4 +58,4 @@ futures = "0.3"
 
 [package.metadata.docs.rs]
 all-features = true
-rustdoc-args = ["--cfg", "docsrs"]
+rustdoc-args = ["--cfg", "docsrs"]

console-subscriber/src/builder.rs

@@ -39,6 +39,9 @@ pub struct Builder {
 
     /// The filter environment variable to use for `tracing` events.
     pub(super) filter_env_var: String,
+
+    /// Whether to trace events coming from the subscriber thread
+    self_trace: bool,
 }
 
 impl Default for Builder {
@@ -51,6 +54,7 @@ impl Default for Builder {
             server_addr: SocketAddr::new(Server::DEFAULT_IP, Server::DEFAULT_PORT),
             recording_path: None,
             filter_env_var: "RUST_LOG".to_string(),
+            self_trace: false,
         }
     }
 }
@@ -176,6 +180,13 @@ impl Builder {
         }
     }
 
+    /// Sets whether we are are tracing events coming from the console subscriber
+    ///
+    /// The default is to drop events coming from the console subscriber thread.
+    pub fn enable_self_trace(self, self_trace: bool) -> Self {
+        Self { self_trace, ..self }
+    }
+
     /// Completes the builder, returning a [`ConsoleLayer`] and [`Server`] task.
     pub fn build(self) -> (ConsoleLayer, Server) {
         ConsoleLayer::build(self)
@@ -368,6 +379,8 @@ impl Builder {
             meta.name().starts_with("runtime.") || meta.target().starts_with("tokio")
         }
 
+        let self_trace = self.self_trace;
+
         let (layer, server) = self.build();
         let filter =
             FilterFn::new(console_filter as for<'r, 's> fn(&'r tracing::Metadata<'s>) -> bool);
@@ -376,6 +389,12 @@ impl Builder {
         thread::Builder::new()
             .name("console_subscriber".into())
             .spawn(move || {
+                let _subscriber_guard;
+                if !self_trace {
+                    _subscriber_guard = tracing::subscriber::set_default(
+                        tracing_core::subscriber::NoSubscriber::default(),
+                    );
+                }
                 let runtime = runtime::Builder::new_current_thread()
                     .enable_io()
                     .enable_time()