Add telemetry for 'bad' experiences based on tag helpers (#10906)

Helps add telemetry for cases like
[AB#2255138](https://devdiv.visualstudio.com/DevDiv/_workitems/edit/2255138/)

src/Razor/src/Microsoft.AspNetCore.Razor.LanguageServer/Diagnostics/DocumentPullDiagnosticsEndpoint.cs

@@ -3,12 +3,16 @@
 
 using System;
 using System.Collections.Generic;
+using System.Collections.Immutable;
+using System.Linq;
 using System.Threading;
 using System.Threading.Tasks;
 using Microsoft.AspNetCore.Razor.Language;
+using Microsoft.AspNetCore.Razor.Language.Components;
 using Microsoft.AspNetCore.Razor.LanguageServer.EndpointContracts;
 using Microsoft.AspNetCore.Razor.LanguageServer.Hosting;
 using Microsoft.AspNetCore.Razor.PooledObjects;
+using Microsoft.AspNetCore.Razor.ProjectSystem;
 using Microsoft.AspNetCore.Razor.Telemetry;
 using Microsoft.CodeAnalysis.Razor.Diagnostics;
 using Microsoft.CodeAnalysis.Razor.ProjectSystem;
@@ -26,6 +30,7 @@ internal class DocumentPullDiagnosticsEndpoint : IRazorRequestHandler<VSInternal
     private readonly IClientConnection _clientConnection;
     private readonly RazorTranslateDiagnosticsService _translateDiagnosticsService;
     private readonly ITelemetryReporter? _telemetryReporter;
+    private ImmutableDictionary<ProjectKey, int> _lastReporedProjectTagHelperCount = ImmutableDictionary<ProjectKey, int>.Empty;
 
     public DocumentPullDiagnosticsEndpoint(
         LanguageServerFeatureOptions languageServerFeatureOptions,
@@ -77,6 +82,8 @@ public TextDocumentIdentifier GetTextDocumentIdentifier(VSInternalDocumentDiagno
 
         var razorDiagnostics = await GetRazorDiagnosticsAsync(documentSnapshot).ConfigureAwait(false);
 
+        await ReportRZ10012TelemetryAsync(documentContext, razorDiagnostics, cancellationToken).ConfigureAwait(false);
+
         var (csharpDiagnostics, htmlDiagnostics) = await GetHtmlCSharpDiagnosticsAsync(documentContext, correlationId, cancellationToken).ConfigureAwait(false);
 
         var diagnosticCount =
@@ -163,4 +170,59 @@ public TextDocumentIdentifier GetTextDocumentIdentifier(VSInternalDocumentDiagno
 
         return (delegatedResponse.CSharpDiagnostics, delegatedResponse.HtmlDiagnostics);
     }
+
+    /// <summary>
+    /// Reports telemetry for RZ10012 "Found markup element with unexpected name" to help track down potential issues
+    /// with taghelpers being discovered (or lack thereof)
+    /// </summary>
+    private async ValueTask ReportRZ10012TelemetryAsync(DocumentContext documentContext, VSInternalDiagnosticReport[]? razorDiagnostics, CancellationToken cancellationToken)
+    {
+        if (razorDiagnostics is null)
+        {
+            return;
+        }
+
+        if (_telemetryReporter is null)
+        {
+            return;
+        }
+
+        var relevantDiagnosticsCount = razorDiagnostics.Sum(CountDiagnostics);
+        if (relevantDiagnosticsCount == 0)
+        {
+            return;
+        }
+
+        var tagHelpers = await documentContext.Project.GetTagHelpersAsync(cancellationToken).ConfigureAwait(false);
+        var tagHelperCount = tagHelpers.Count();
+        var shouldReport = false;
+
+        ImmutableInterlocked.AddOrUpdate(
+            ref _lastReporedProjectTagHelperCount,
+            documentContext.Project.Key,
+            (k) =>
+            {
+                shouldReport = true;
+                return tagHelperCount;
+            },
+            (k, currentValue) =>
+            {
+                shouldReport = currentValue != tagHelperCount;
+                return tagHelperCount;
+            });
+
+        if (shouldReport)
+        {
+            _telemetryReporter.ReportEvent(
+                "RZ10012",
+                Severity.Low,
+                new("tagHelpers", tagHelperCount),
+                new("RZ10012errors", relevantDiagnosticsCount),
+                new("project", documentContext.Project.Key.Id));
+        }
+
+        static int CountDiagnostics(VSInternalDiagnosticReport report)
+            => report.Diagnostics?.Count(d => d.Code == ComponentDiagnosticFactory.UnexpectedMarkupElement.Id)
+            ?? 0;
+    }
 }