subscriber: add Targets::default_level method (#2242)

## Motivation

This makes it possible to fully "override" some base `Targets` filter
with another (e.g. user-supplied) filter. Without some way to obtain the
default, only explicit targets can be overridden (via `IntoIter` and
friends). 

See also tokio-rs/tracing#1790 (comment)

## Solution

We can add a method to `Targets` that filters the underlying
`DirectiveSet` for the default directive. This works because
`DirectiveSet::add` will replace directives with the same
`target`/`field_names`, which is always `None`/`vec![]` for the
directive added by `with_default` (and in fact we are only concerned
with `target`, since no other `Targets` API allows adding directives
with a `None` target).

Ideally the method would be named `default`, corresponding to
`with_default`, however this conflicts with `Default::default` and so
would be a breaking change (and harm ergonomics). `default_level` seemed
a better name than `get_default`, since "getters" of this style are
generally considered unidiomatic<sup>[citation needed]</sup>.

Example usage:

```rust
let mut filter = Targets::new().with_target("some_module", LevelFilter::INFO);

// imagine this came from `RUST_LOG` or similar
let override: Targets = "trace".parse().unwrap();

// merge the override
if let Some(default) = override.default_level() {
    filter = filter.with_default(default);
}
for (target, level) in override.iter() {
    filter = filter.with_target(target, level);
}
```

Closes #1790

tracing-subscriber/src/filter/targets.rs

@@ -277,6 +277,62 @@ impl Targets {
         self
     }
 
+    /// Returns the default level for this filter, if one is set.
+    ///
+    /// The default level is used to filter any spans or events with targets
+    /// that do not match any of the configured set of prefixes.
+    ///
+    /// The default level can be set for a filter either by using
+    /// [`with_default`](Self::with_default) or when parsing from a filter string that includes a
+    /// level without a target (e.g. `"trace"`).
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use tracing_subscriber::filter::{LevelFilter, Targets};
+    ///
+    /// let filter = Targets::new().with_default(LevelFilter::INFO);
+    /// assert_eq!(filter.default_level(), Some(LevelFilter::INFO));
+    ///
+    /// let filter: Targets = "info".parse().unwrap();
+    /// assert_eq!(filter.default_level(), Some(LevelFilter::INFO));
+    /// ```
+    ///
+    /// The default level is `None` if no default is set:
+    ///
+    /// ```
+    /// use tracing_subscriber::filter::Targets;
+    ///
+    /// let filter = Targets::new();
+    /// assert_eq!(filter.default_level(), None);
+    ///
+    /// let filter: Targets = "my_crate=info".parse().unwrap();
+    /// assert_eq!(filter.default_level(), None);
+    /// ```
+    ///
+    /// Note that an unset default level (`None`) behaves like [`LevelFilter::OFF`] when the filter is
+    /// used, but it could also be set explicitly which may be useful to distinguish (such as when
+    /// merging multiple `Targets`).
+    ///
+    /// ```
+    /// use tracing_subscriber::filter::{LevelFilter, Targets};
+    ///
+    /// let filter = Targets::new().with_default(LevelFilter::OFF);
+    /// assert_eq!(filter.default_level(), Some(LevelFilter::OFF));
+    ///
+    /// let filter: Targets = "off".parse().unwrap();
+    /// assert_eq!(filter.default_level(), Some(LevelFilter::OFF));
+    /// ```
+    pub fn default_level(&self) -> Option<LevelFilter> {
+        self.0.directives().into_iter().find_map(|d| {
+            if d.target.is_none() {
+                Some(d.level)
+            } else {
+                None
+            }
+        })
+    }
+
     /// Returns an iterator over the [target]-[`LevelFilter`] pairs in this filter.
     ///
     /// The order of iteration is undefined.
@@ -685,6 +741,21 @@ mod tests {
         );
     }
 
+    #[test]
+    fn targets_default_level() {
+        let filter = expect_parse("crate1::mod1=error,crate1::mod2,crate2=debug,crate3=off");
+        assert_eq!(filter.default_level(), None);
+
+        let filter = expect_parse("crate1::mod1=error,crate1::mod2,crate2=debug,crate3=off")
+            .with_default(LevelFilter::OFF);
+        assert_eq!(filter.default_level(), Some(LevelFilter::OFF));
+
+        let filter = expect_parse("crate1::mod1=error,crate1::mod2,crate2=debug,crate3=off")
+            .with_default(LevelFilter::OFF)
+            .with_default(LevelFilter::INFO);
+        assert_eq!(filter.default_level(), Some(LevelFilter::INFO));
+    }
+
     #[test]
     // `println!` is only available with `libstd`.
     #[cfg(feature = "std")]