subscriber: mark builders as must_use (#2239)

## Motivation

Builders not marked `#[must_use]` can not be initialized sometimes,
causing silent failures.
Eg.
```rust
fn main() {
    tracing_subscriber::fmt();
    tracing::info!("hello");
}
```
won't print anything.

## Solution

Added `#[must_use]` to builder types in the tracing-subscriber crate.

tracing-subscriber/src/filter/env/builder.rs

@@ -11,6 +11,7 @@ use tracing::level_filters::STATIC_MAX_LEVEL;
 ///
 /// [builder]: https://rust-unofficial.github.io/patterns/patterns/creational/builder.html
 #[derive(Debug, Clone)]
+#[must_use]
 pub struct Builder {
     regex: bool,
     env: Option<String>,

tracing-subscriber/src/fmt/mod.rs

@@ -243,6 +243,7 @@ pub type Formatter<
 /// Configures and constructs `Subscriber`s.
 #[cfg_attr(docsrs, doc(cfg(all(feature = "fmt", feature = "std"))))]
 #[derive(Debug)]
+#[must_use]
 pub struct SubscriberBuilder<
     N = format::DefaultFields,
     E = format::Format<format::Full>,
@@ -465,7 +466,8 @@ impl Default for SubscriberBuilder {
         SubscriberBuilder {
             filter: Subscriber::DEFAULT_MAX_LEVEL,
             inner: Default::default(),
-        }.log_internal_errors(true)
+        }
+        .log_internal_errors(true)
     }
 }
 
@@ -631,7 +633,10 @@ where
     /// as a fallback.
     ///
     /// [`FormatEvent`]: crate::fmt::FormatEvent
-    pub fn log_internal_errors(self, log_internal_errors: bool) -> SubscriberBuilder<N, format::Format<L, T>, F, W> {
+    pub fn log_internal_errors(
+        self,
+        log_internal_errors: bool,
+    ) -> SubscriberBuilder<N, format::Format<L, T>, F, W> {
         SubscriberBuilder {
             inner: self.inner.log_internal_errors(log_internal_errors),
             ..self