feat(log): Log the tracing level when it is set or reloaded (#4515)

* Log the tracing level when it is set or reloaded

* Add the `log` crate as a direct dependency

* Make `log` required

Cargo.lock

@@ -6520,6 +6520,7 @@ dependencies = [
  "indexmap",
  "inferno",
  "lazy_static",
+ "log",
  "metrics",
  "metrics-exporter-prometheus",
  "num-integer",

zebrad/Cargo.toml

@@ -75,6 +75,10 @@ rand = { version = "0.8.5", package = "rand" }
 sentry-tracing = { version = "0.23.0", optional = true }
 sentry = { version = "0.23.0", default-features = false, features = ["backtrace", "contexts", "reqwest", "rustls"], optional = true }
 
+# zebrad uses tracing for logging,
+# we only use `log` to print the static log levels in transitive dependencies
+log = "0.4.14"
+
 # test feature proptest-impl
 proptest = { version = "0.10.1", optional = true }
 proptest-derive = { version = "0.3.0", optional = true }

zebrad/src/components/tracing/component.rs

@@ -31,7 +31,7 @@ impl Tracing {
         // Construct a tracing subscriber with the supplied filter and enable reloading.
         let builder = FmtSubscriber::builder()
             .with_ansi(use_color)
-            .with_env_filter(filter)
+            .with_env_filter(&filter)
             .with_filter_reloading();
         let filter_handle = builder.reload_handle();
 
@@ -62,6 +62,13 @@ impl Tracing {
             (Some(layer1), Some(layer2)) => subscriber.with(layer1).with(layer2).init(),
         };
 
+        tracing::info!(
+            ?filter,
+            TRACING_STATIC_MAX_LEVEL = ?tracing::level_filters::STATIC_MAX_LEVEL,
+            LOG_STATIC_MAX_LEVEL = ?log::STATIC_MAX_LEVEL,
+            "started tracing component",
+        );
+
         Ok(Self {
             filter_handle,
             flamegrapher,
@@ -79,9 +86,19 @@ impl Tracing {
     ///
     /// This can be used to provide a dynamic tracing filter endpoint.
     pub fn reload_filter(&self, filter: impl Into<EnvFilter>) {
+        let filter = filter.into();
+        let filter_str = filter.to_string();
+
         self.filter_handle
             .reload(filter)
             .expect("the subscriber is not dropped before the component is");
+
+        tracing::info!(
+            filter = ?filter_str,
+            TRACING_STATIC_MAX_LEVEL = ?tracing::level_filters::STATIC_MAX_LEVEL,
+            LOG_STATIC_MAX_LEVEL = ?log::STATIC_MAX_LEVEL,
+            "reloaded tracing filter",
+        );
     }
 }