subscriber: fix FmtCollector not forwarding max level (#1251)

The `FmtCollector` type is missing a `Collect::max_level_hint
method that forwards the inner stack's max level hint.

This fixes that, and adds tests.

Signed-off-by: Eliza Weisman <eliza@buoyant.io>

tracing-subscriber/src/fmt/mod.rs

@@ -489,6 +489,11 @@ where
         self.inner.try_close(id)
     }
 
+    #[inline]
+    fn max_level_hint(&self) -> Option<tracing_core::LevelFilter> {
+        self.inner.max_level_hint()
+    }
+
     unsafe fn downcast_raw(&self, id: TypeId) -> Option<*const ()> {
         if id == TypeId::of::<Self>() {
             Some(self as *const Self as *const ())

tracing-subscriber/tests/fmt_max_level_hint.rs

@@ -0,0 +1,9 @@
+use tracing_subscriber::filter::LevelFilter;
+
+#[test]
+fn fmt_sets_max_level_hint() {
+    tracing_subscriber::fmt()
+        .with_max_level(LevelFilter::DEBUG)
+        .init();
+    assert_eq!(LevelFilter::current(), LevelFilter::DEBUG);
+}

tracing-subscriber/tests/registry_max_level_hint.rs

@@ -0,0 +1,10 @@
+use tracing_subscriber::{filter::LevelFilter, prelude::*};
+
+#[test]
+fn registry_sets_max_level_hint() {
+    tracing_subscriber::registry()
+        .with(tracing_subscriber::fmt::layer())
+        .with(LevelFilter::DEBUG)
+        .init();
+    assert_eq!(LevelFilter::current(), LevelFilter::DEBUG);
+}