make AggregatorHandle abort explcit

Instead of aborting the aggregator task upon dropping the
`AggregatorHandle`, we now provide an explicit `abort` method. If the
user chooses, they can discard the aggregator handle and while they will
lose the possibility to clean up the aggregator task, they will be
otherwise unaffected.

console-subscriber/src/lib.rs

@@ -1000,7 +1000,8 @@ impl Server {
     /// tokio::spawn(serve);
     /// # // Avoid a warning that `console_layer` and `aggregator_handle` are unused.
     /// # drop(console_layer);
-    /// # drop(aggregator_handle);
+    /// # let mut aggregator_handle = aggregator_handle;
+    /// # aggregator_handle.abort();
     /// # });
     /// ```
     ///
@@ -1031,8 +1032,8 @@ impl Server {
 /// The `InstrumentServer<Server>` can be used to construct a router which
 /// can be added to a [`tonic`] gRPC server.
 ///
-/// The [`AggregatorHandle`] must be kept until after the server has been
-/// shut down.
+/// The [`AggregatorHandle`] can be used to abort the associated aggregator task
+/// after the server has been shut down.
 ///
 /// See the [`Server::into_parts`] documentation for usage.
 #[non_exhaustive]
@@ -1050,15 +1051,32 @@ pub struct ServerParts {
 
 /// Aggregator handle.
 ///
-/// This object is returned from [`Server::into_parts`] and must be
-/// kept as long as the `InstrumentServer<Server>` - which is also
-/// returned - is in use.
+/// This object is returned from [`Server::into_parts`]. It can be
+/// used to abort the aggregator task.
+///
+/// The aggregator collects the traces that implement the async runtime
+/// being observed and prepares them to be served by the gRPC server.
+///
+/// Normally, if the server, started with [`Server::serve`] or
+/// [`Server::serve_with`] stops for any reason, the aggregator is aborted,
+/// hoewver, if the server was started with the [`InstrumentServer`] returned
+/// from [`Server::into_parts`], then it is the responsibility of the user
+/// of the API to stop the aggregator task by calling [`abort`] on this
+/// object.
+///
+/// [`abort`]: fn@crate::Aggregator::abort
 pub struct AggregatorHandle {
     join_handle: JoinHandle<()>,
 }
 
-impl Drop for AggregatorHandle {
-    fn drop(&mut self) {
+impl AggregatorHandle {
+    /// Aborts the task running this aggregator.
+    ///
+    /// To avoid having a disconnected aggregator running forever, this
+    /// method should be called when the [`tonic::transport::Server`] started
+    /// with the [`InstrumentServer`] also returned from [`Server::into_parts`]
+    /// stops running.
+    pub fn abort(&mut self) {
         self.join_handle.abort();
     }
 }