[analyzer server] Legacy protocol debounces hover requests too

TL;DR: By only responding (for real) to the newest hover request a
benchmark that used to take ~10 seconds now takes ~0.6 seconds.

Details:

In #55281 (comment)
I noticed that some of the performance issues people are seeing fom the
analyzer server stem from many requests being fired, each taking longer
the wait before the next is issued, naturally building up a longer and
loger queue before responses are made. Their it was for `edit.getFixes`,
but I'm yet to actually reproduce that. I did reproduce for hover
requests though, so here I'm debounsing those hover requests.
I'm hooking into the system that already debounces completion requests
and following the pattern used there.

On my machine, running

```
out/ReleaseX64/dart-sdk/bin/dart \
pkg/analysis_server/tool/benchmark_tools/big_chain_benchmark/legacy_many_hover_requests.dart
```

I get these numbers:

Before CL (taken from the parent CL that introduces the benchmark):

```
4 files / CodeType.ImportCycle:
Initial analysis: 1.249438
Completion after change: 10.143719

4 files / CodeType.ImportChain:
Initial analysis: 1.367604
Completion after change: 9.936909

4 files / CodeType.ImportExportCycle:
Initial analysis: 1.233644
Completion after change: 10.011695

4 files / CodeType.ImportExportChain:
Initial analysis: 1.226382
Completion after change: 9.875991

4 files / CodeType.ImportCycleExportChain:
Initial analysis: 1.262932
Completion after change: 9.995607
```

With CL:

```
4 files / CodeType.ImportCycle:
Initial analysis: 1.322501
Completion after change: 0.569768

4 files / CodeType.ImportChain:
Initial analysis: 1.347588
Completion after change: 0.635748

4 files / CodeType.ImportExportCycle:
Initial analysis: 1.492362
Completion after change: 0.575233

4 files / CodeType.ImportExportChain:
Initial analysis: 1.331619
Completion after change: 0.576199

4 files / CodeType.ImportCycleExportChain:
Initial analysis: 1.342130
Completion after change: 0.608064
```

Change-Id: Ide3dbc4dfb885af9e1c46a763338ba545c0ab7db
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/413982
Reviewed-by: Phil Quitslund <[email protected]>
Commit-Queue: Jens Johansen <[email protected]>

Author: jensjoha

pkg/analysis_server/lib/src/server/debounce_requests.dart

@@ -16,6 +16,7 @@ import 'package:analysis_server/src/channel/channel.dart';
 /// then most probably the user continued typing, and there is no need to
 /// compute results for the first request. But we will have to respond, an
 /// empty response is enough.
+/// The same goes for hover requests.
 ///
 /// Discarded requests are reported into [discardedRequests].
 Stream<RequestOrResponse> debounceRequests(
@@ -44,7 +45,7 @@ class _DebounceRequests {
           // quickly get all of them. So, even 1 ms should be enough.
           timer ??= Timer(const Duration(milliseconds: 1), () {
             timer = null;
-            var filtered = _filterCompletion(buffer);
+            var filtered = _filterRequests(buffer);
             buffer = [];
             for (var request in filtered) {
               sink.add(request);
@@ -55,15 +56,16 @@ class _DebounceRequests {
     );
   }
 
-  List<RequestOrResponse> _filterCompletion(List<RequestOrResponse> requests) {
+  List<RequestOrResponse> _filterRequests(List<RequestOrResponse> requests) {
     var reversed = <RequestOrResponse>[];
     var abortCompletionRequests = false;
+    var abortHoverRequests = false;
     for (var requestOrResponse in requests.reversed) {
       if (requestOrResponse is Request) {
         if (requestOrResponse.method == ANALYSIS_REQUEST_UPDATE_CONTENT) {
           abortCompletionRequests = true;
-        }
-        if (requestOrResponse.method == COMPLETION_REQUEST_GET_SUGGESTIONS2) {
+        } else if (requestOrResponse.method ==
+            COMPLETION_REQUEST_GET_SUGGESTIONS2) {
           if (abortCompletionRequests) {
             discardedRequests.add(requestOrResponse);
             var params = CompletionGetSuggestions2Params.fromRequest(
@@ -85,6 +87,21 @@ class _DebounceRequests {
           } else {
             abortCompletionRequests = true;
           }
+        } else if (requestOrResponse.method == ANALYSIS_REQUEST_GET_HOVER) {
+          if (abortHoverRequests) {
+            discardedRequests.add(requestOrResponse);
+            channel.sendResponse(
+              AnalysisGetHoverResult([]).toResponse(
+                requestOrResponse.id,
+                // We can use a null converter here because we're not sending
+                // any path.
+                clientUriConverter: null,
+              ),
+            );
+            continue;
+          } else {
+            abortHoverRequests = true;
+          }
         }
       }
       reversed.add(requestOrResponse);