[R&D] Fast-path bail in Activity intercepts when interception disabled

Without this, every Activity.SetTag / AddTag / set_DisplayName / SetStatus
/ getter call across the host process — for users who haven't opted into
interception — would still hit Activity.GetCustomProperty("__dd_span__")
in the integration's OnMethodBegin. That's a Dictionary lookup on every
intercepted call, paid by every modern .NET 6+ process whose JIT applies
our CallTarget rewrites.

Add `if (!IsActivityInterceptionEnabled) return GetDefault();` at the top
of each Activity-direct intercept (ActivityStartIntegration and
ActivityStopIntegration already had it). Brings non-interception per-call
cost down to a single bool field read.

Verified across the full Windows TFM matrix that interception tests
(SubmitsTracesWithInterception) still pass on net48, netcoreapp3.1,
net6.0, net7.0, net8.0, net9.0, net10.0; and listener-path tests
(SubmitsTraces, SubmitsTracesWithActivitySource) still pass on net6.0.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: andrewlock

tracer/src/Datadog.Trace/ClrProfiler/AutoInstrumentation/Activity/ActivityAddTagObjectIntegration.cs

@@ -40,6 +40,12 @@ public sealed class ActivityAddTagObjectIntegration
         /// </summary>
         internal static CallTargetState OnMethodBegin<TTarget, TKey, TValue>(TTarget instance, TKey key, TValue value)
         {
+            // Fast-path bail when interception isn't enabled — see ActivitySetTagIntegration for rationale.
+            if (!Tracer.Instance.Settings.IsActivityInterceptionEnabled)
+            {
+                return CallTargetState.GetDefault();
+            }
+
             var scope = ActivityCustomPropertyAccessor<TTarget>.GetScope(instance);
             if (scope is not null)
             {

tracer/src/Datadog.Trace/ClrProfiler/AutoInstrumentation/Activity/ActivityAddTagStringIntegration.cs

@@ -39,6 +39,12 @@ public sealed class ActivityAddTagStringIntegration
         /// </summary>
         internal static CallTargetState OnMethodBegin<TTarget, TKey, TValue>(TTarget instance, TKey key, TValue value)
         {
+            // Fast-path bail when interception isn't enabled — see ActivitySetTagIntegration for rationale.
+            if (!Tracer.Instance.Settings.IsActivityInterceptionEnabled)
+            {
+                return CallTargetState.GetDefault();
+            }
+
             var scope = ActivityCustomPropertyAccessor<TTarget>.GetScope(instance);
             if (scope is not null)
             {

tracer/src/Datadog.Trace/ClrProfiler/AutoInstrumentation/Activity/ActivityDisplayNameGetterIntegration.cs

@@ -39,6 +39,12 @@ public sealed class ActivityDisplayNameGetterIntegration
         /// </summary>
         internal static CallTargetState OnMethodBegin<TTarget>(TTarget instance)
         {
+            // Fast-path bail when interception isn't enabled — see ActivitySetTagIntegration for rationale.
+            if (!Tracer.Instance.Settings.IsActivityInterceptionEnabled)
+            {
+                return CallTargetState.GetDefault();
+            }
+
             var scope = ActivityCustomPropertyAccessor<TTarget>.GetScope(instance);
             if (scope is not null)
             {

tracer/src/Datadog.Trace/ClrProfiler/AutoInstrumentation/Activity/ActivityDisplayNameIntegration.cs

@@ -38,6 +38,12 @@ public sealed class ActivityDisplayNameIntegration
         /// </summary>
         internal static CallTargetState OnMethodBegin<TTarget, TValue>(TTarget instance, TValue value)
         {
+            // Fast-path bail when interception isn't enabled — see ActivitySetTagIntegration for rationale.
+            if (!Tracer.Instance.Settings.IsActivityInterceptionEnabled)
+            {
+                return CallTargetState.GetDefault();
+            }
+
             var scope = ActivityCustomPropertyAccessor<TTarget>.GetScope(instance);
             if (scope is not null)
             {

tracer/src/Datadog.Trace/ClrProfiler/AutoInstrumentation/Activity/ActivitySetStatusIntegration.cs

@@ -43,6 +43,12 @@ public sealed class ActivitySetStatusIntegration
         /// <typeparam name="TDescription">string type</typeparam>
         internal static CallTargetState OnMethodBegin<TTarget, TStatusCode, TDescription>(TTarget instance, TStatusCode statusCode, TDescription description)
         {
+            // Fast-path bail when interception isn't enabled — see ActivitySetTagIntegration for rationale.
+            if (!Tracer.Instance.Settings.IsActivityInterceptionEnabled)
+            {
+                return CallTargetState.GetDefault();
+            }
+
             var scope = ActivityCustomPropertyAccessor<TTarget>.GetScope(instance);
             if (scope is not null)
             {

tracer/src/Datadog.Trace/ClrProfiler/AutoInstrumentation/Activity/ActivitySetTagIntegration.cs

@@ -40,6 +40,14 @@ public sealed class ActivitySetTagIntegration
         /// </summary>
         internal static CallTargetState OnMethodBegin<TTarget, TKey, TValue>(TTarget instance, TKey key, TValue value)
         {
+            // Fast-path bail when interception isn't enabled. Without this check, every Activity.SetTag
+            // call across the host process would hit the Activity custom-property lookup on the next
+            // line — pure overhead for users who haven't opted into the interception path.
+            if (!Tracer.Instance.Settings.IsActivityInterceptionEnabled)
+            {
+                return CallTargetState.GetDefault();
+            }
+
             var scope = ActivityCustomPropertyAccessor<TTarget>.GetScope(instance);
             if (scope is not null)
             {

tracer/src/Datadog.Trace/ClrProfiler/AutoInstrumentation/Activity/ActivityStatusDescriptionGetterIntegration.cs

@@ -40,6 +40,12 @@ public sealed class ActivityStatusDescriptionGetterIntegration
         /// </summary>
         internal static CallTargetState OnMethodBegin<TTarget>(TTarget instance)
         {
+            // Fast-path bail when interception isn't enabled — see ActivitySetTagIntegration for rationale.
+            if (!Tracer.Instance.Settings.IsActivityInterceptionEnabled)
+            {
+                return CallTargetState.GetDefault();
+            }
+
             var scope = ActivityCustomPropertyAccessor<TTarget>.GetScope(instance);
             if (scope is not null)
             {

tracer/src/Datadog.Trace/ClrProfiler/AutoInstrumentation/Activity/ActivityStatusGetterIntegration.cs

@@ -41,6 +41,12 @@ public sealed class ActivityStatusGetterIntegration
         /// </summary>
         internal static CallTargetState OnMethodBegin<TTarget>(TTarget instance)
         {
+            // Fast-path bail when interception isn't enabled — see ActivitySetTagIntegration for rationale.
+            if (!Tracer.Instance.Settings.IsActivityInterceptionEnabled)
+            {
+                return CallTargetState.GetDefault();
+            }
+
             var scope = ActivityCustomPropertyAccessor<TTarget>.GetScope(instance);
             if (scope is not null)
             {

tracer/src/Datadog.Trace/ClrProfiler/AutoInstrumentation/Activity/ActivityTagObjectsGetterIntegration.cs

@@ -43,6 +43,12 @@ public sealed class ActivityTagObjectsGetterIntegration
         /// </summary>
         internal static CallTargetState OnMethodBegin<TTarget>(TTarget instance)
         {
+            // Fast-path bail when interception isn't enabled — see ActivitySetTagIntegration for rationale.
+            if (!Tracer.Instance.Settings.IsActivityInterceptionEnabled)
+            {
+                return CallTargetState.GetDefault();
+            }
+
             var scope = ActivityCustomPropertyAccessor<TTarget>.GetScope(instance);
             if (scope is not null)
             {

tracer/src/Datadog.Trace/ClrProfiler/AutoInstrumentation/Activity/ActivityTagsGetterIntegration.cs

@@ -42,6 +42,12 @@ public sealed class ActivityTagsGetterIntegration
         /// </summary>
         internal static CallTargetState OnMethodBegin<TTarget>(TTarget instance)
         {
+            // Fast-path bail when interception isn't enabled — see ActivitySetTagIntegration for rationale.
+            if (!Tracer.Instance.Settings.IsActivityInterceptionEnabled)
+            {
+                return CallTargetState.GetDefault();
+            }
+
             var scope = ActivityCustomPropertyAccessor<TTarget>.GetScope(instance);
             if (scope is not null)
             {