Single-span web spans PoC

tracer/build/crank/Samples.AspNetCoreSimpleController.yml

@@ -43,6 +43,7 @@ scenarios:
         CORECLR_ENABLE_PROFILING: 1
         DD_CLR_ENABLE_INLINING: 1
         DD_CLR_ENABLE_NGEN: 1
+        DD_TRACE_SINGLE_WEB_SPAN_ENABLED: "1"
     load:
       job: bombardier
       variables:

tracer/src/Datadog.Trace/ClrProfiler/Instrumentation.cs

@@ -432,12 +432,16 @@ private static void StartDiagnosticManager()
         {
             var observers = new List<DiagnosticObserver>();
 
-            if (Tracer.Instance.Settings.AzureAppServiceMetadata?.IsFunctionsApp is not true)
+            var settings = Tracer.Instance.Settings;
+            if (settings.AzureAppServiceMetadata?.IsFunctionsApp is not true)
             {
                 // Not adding the `AspNetCoreDiagnosticObserver` is particularly important for Azure Functions.
                 // The AspNetCoreDiagnosticObserver will be loaded in a separate Assembly Load Context, breaking the connection of AsyncLocal
                 // This is because user code is loaded within the functions host in a separate context
-                observers.Add(new AspNetCoreDiagnosticObserver());
+                DiagnosticObserver observer = settings.SingleWebSpanEnabled
+                                                            ? new SingleSpanAspNetCoreDiagnosticObserver()
+                                                            : new AspNetCoreDiagnosticObserver();
+                observers.Add(observer);
             }
 
             var diagnosticManager = new DiagnosticManager(observers);

tracer/src/Datadog.Trace/Configuration/ConfigurationKeys.cs

@@ -578,6 +578,14 @@ internal static class FeatureFlags
             /// <seealso cref="TracerSettings.RouteTemplateResourceNamesEnabled"/>
             public const string RouteTemplateResourceNamesEnabled = "DD_TRACE_ROUTE_TEMPLATE_RESOURCE_NAMES_ENABLED";
 
+            /// <summary>
+            /// When enabled, <c>aspnet_core_mvc.request</c>, <c>aspnet-webapi.request</c>, and <c>aspnet-mvc.request</c>
+            /// spans will not be created. Instead, tags typically associated with these spans will be associated with
+            /// the <c>aspnet.request</c> and <c>aspnet_core.request</c> span instead.
+            /// </summary>
+            /// <seealso cref="TracerSettings.SingleWebSpanEnabled"/>
+            public const string SingleWebSpanEnabled = "DD_TRACE_SINGLE_WEB_SPAN_ENABLED";
+
             /// <summary>
             /// Configuration key to enable or disable the updated WCF instrumentation that delays execution
             /// until later in the WCF pipeline when the WCF server exception handling is established.

tracer/src/Datadog.Trace/Configuration/ImmutableTracerSettings.cs

@@ -122,6 +122,7 @@ internal ImmutableTracerSettings(TracerSettings settings, bool unusedParamNotToU
             TraceBufferSize = settings.TraceBufferSize;
             TraceBatchInterval = settings.TraceBatchInterval;
             RouteTemplateResourceNamesEnabled = settings.RouteTemplateResourceNamesEnabled;
+            SingleWebSpanEnabled = settings.SingleWebSpanEnabled;
             DelayWcfInstrumentationEnabled = settings.DelayWcfInstrumentationEnabled;
             WcfWebHttpResourceNamesEnabled = settings.WcfWebHttpResourceNamesEnabled;
             WcfObfuscationEnabled = settings.WcfObfuscationEnabled;
@@ -426,6 +427,12 @@ internal ImmutableTracerSettings(TracerSettings settings, bool unusedParamNotToU
         /// <seealso cref="ConfigurationKeys.FeatureFlags.RouteTemplateResourceNamesEnabled"/>
         internal bool RouteTemplateResourceNamesEnabled { get; }
 
+        /// <summary>
+        /// Gets a value indicating whether the feature flag to enable single-web spans is enabled
+        /// </summary>
+        /// <seealso cref="ConfigurationKeys.FeatureFlags.SingleWebSpanEnabled"/>
+        internal bool SingleWebSpanEnabled { get; }
+
         /// <summary>
         /// Gets a value indicating whether route parameters in ASP.NET and ASP.NET Core resource names
         /// should be expanded with their values. Only applies when  <see cref="RouteTemplateResourceNamesEnabled"/>

tracer/src/Datadog.Trace/Configuration/TracerSettings.cs

@@ -236,6 +236,11 @@ internal TracerSettings(IConfigurationSource? source, IConfigurationTelemetry te
                                                .WithKeys(ConfigurationKeys.FeatureFlags.RouteTemplateResourceNamesEnabled)
                                                .AsBool(defaultValue: true);
 
+            SingleWebSpanEnabled = RouteTemplateResourceNamesEnabled // always disabled if route template resources names disabled
+                                && config
+                                  .WithKeys(ConfigurationKeys.FeatureFlags.SingleWebSpanEnabled)
+                                  .AsBool(defaultValue: false);
+
             ExpandRouteTemplatesEnabled = config
                                          .WithKeys(ConfigurationKeys.ExpandRouteTemplatesEnabled)
                                          .AsBool(defaultValue: !RouteTemplateResourceNamesEnabled); // disabled by default if route template resource names enabled
@@ -792,6 +797,12 @@ public bool DiagnosticSourceEnabled
         /// <seealso cref="ConfigurationKeys.FeatureFlags.RouteTemplateResourceNamesEnabled"/>
         internal bool RouteTemplateResourceNamesEnabled { get; }
 
+        /// <summary>
+        /// Gets a value indicating whether the feature flag to enable single-web spans is enabled
+        /// </summary>
+        /// <seealso cref="ConfigurationKeys.FeatureFlags.SingleWebSpanEnabled"/>
+        internal bool SingleWebSpanEnabled { get; }
+
         /// <summary>
         /// Gets a value indicating whether resource names for ASP.NET and ASP.NET Core spans should be expanded. Only applies
         /// when <see cref="RouteTemplateResourceNamesEnabled"/> is <code>true</code>.