message: subscriber: add `must_use` for `CollectorBuilder::finish`
  ## Motivation
  Follow up to
  Discovered that
  ```rust
  tracing_subscriber::fmt().with_ansi(false).finish();
  tracing::info!("hello");
  ```
  doesn't actually do anything, because `finish` returns a `Collector` and
  `init` is the thing that actually installs the collector.
  ## Solution
  Added `#[must_use]` which suggests using `try_init` instead.
diff: @@ -476,6 +476,7 @@ where
  subscribe::Subscribe<Registry> + Send + Sync + 'static,
  {
  /// Finish the builder, returning a new `FmtCollector`.
  + #[must_use = "you may want to use `try_init` or similar to actually install the collector."]
  pub fn finish(self) -> Collector<N, E, F, W> {
  let collector = self.inner.with_collector(Registry::default());
  Collector {
  