[ASM] Standalone Billing (part 2: Propagation) (#5743)

## Summary of changes

Part 2 of the implementation of [Standalone ASM
billing](https://docs.google.com/document/d/12NBx-nD-IoQEMiCRnJXneq4Be7cbtSc6pJLOFUWTpNE/edit?pli=1#heading=h.hs091bhdmugz).
Part 1 was #5565 

This PR adds the propagated span tag `_dd.p.appsec: 1` allowing to
propagate to downstream services the information that the current
distributed trace is containing at least one ASM security event.

## Test coverage

This fully pass the system tests for ASM Standalone.

tracer/src/Datadog.Trace/AppSec/Security.cs

@@ -482,6 +482,7 @@ internal void SetTraceSamplingPriority(Span span)
             else if (_rateLimiter?.Allowed(span) ?? false)
             {
                 span.Context.TraceContext?.SetSamplingPriority(SamplingPriorityValues.UserKeep, SamplingMechanism.Asm);
+                span.Context.TraceContext?.Tags.SetTag(Tags.Propagated.AppSec, "1");
             }
         }
 

tracer/src/Datadog.Trace/Iast/IastModule.cs

@@ -562,6 +562,7 @@ private static IastModuleResponse AddWebVulnerability(string? evidenceValue, Int
         {
             traceContext.IastRequestContext?.AddVulnerability(vulnerability);
             traceContext.SetSamplingPriority(SamplingPriorityValues.UserKeep, SamplingMechanism.Asm);
+            traceContext.Tags.SetTag(Tags.Propagated.AppSec, "1");
 
             return IastModuleResponse.Vulnerable;
         }
@@ -636,6 +637,8 @@ private static IastModuleResponse GetScope(string evidenceValue, IntegrationId i
             if (isRequest)
             {
                 traceContext?.SetSamplingPriority(SamplingPriorityValues.UserKeep, SamplingMechanism.Asm);
+                traceContext?.Tags.SetTag(Tags.Propagated.AppSec, "1");
+
                 traceContext?.IastRequestContext?.AddVulnerability(vulnerability);
                 return IastModuleResponse.Vulnerable;
             }
@@ -691,6 +694,7 @@ private static IastModuleResponse AddVulnerabilityAsSingleSpan(Tracer tracer, In
         scope.Span.Type = SpanTypes.IastVulnerability;
         tracer.TracerManager.Telemetry.IntegrationGeneratedSpan(integrationId);
         scope.Span.Context.TraceContext?.SetSamplingPriority(SamplingPriorityValues.UserKeep, SamplingMechanism.Asm);
+        scope.Span.Context.TraceContext?.Tags.SetTag(Tags.Propagated.AppSec, "1");
         return new IastModuleResponse(scope);
     }
 

tracer/src/Datadog.Trace/Propagators/DatadogContextPropagator.cs

@@ -43,7 +43,6 @@ public void Inject<TCarrier, TCarrierSetter>(SpanContext context, TCarrier carri
             }
 
             var propagatedTagsHeader = context.PrepareTagsHeaderForPropagation();
-
             if (!string.IsNullOrEmpty(propagatedTagsHeader))
             {
                 carrierSetter.Set(carrier, HttpHeaderNames.PropagatedTags, propagatedTagsHeader!);

tracer/src/Datadog.Trace/Propagators/SpanContextPropagator.cs

@@ -115,6 +115,12 @@ internal void Inject<TCarrier, TCarrierSetter>(SpanContext context, TCarrier car
             if (context == null!) { ThrowHelper.ThrowArgumentNullException(nameof(context)); }
             if (carrier == null) { ThrowHelper.ThrowArgumentNullException(nameof(carrier)); }
 
+            // If appsec standalone is enabled and appsec propagation is disabled (no ASM events) -> stop propagation
+            if (context.TraceContext?.Tracer.Settings?.AppsecStandaloneEnabledInternal == true && context.TraceContext.Tags.GetTag(Tags.Propagated.AppSec) != "1")
+            {
+                return;
+            }
+
             // trigger a sampling decision if it hasn't happened yet
             _ = context.GetOrMakeSamplingDecision();
 

tracer/src/Datadog.Trace/Tags.cs

@@ -725,6 +725,12 @@ internal static class Propagated
             /// lower-case hexadecimal string with no zero-padding or `0x` prefix.
             /// </summary>
             internal const string TraceIdUpper = "_dd.p.tid";
+
+            /// <summary>
+            /// A boolean allowing the propagation to downstream services the information that the current distributed trace
+            /// is containing at least one ASM security event, no matter its type (threats, business logic events, IAST, etc.).
+            /// </summary>
+            internal const string AppSec = "_dd.p.appsec";
         }
     }
 }

tracer/src/Datadog.Trace/Tracer.cs

@@ -419,13 +419,25 @@ internal SpanContext CreateSpanContext(ISpanContext parent = null, string servic
 
                 if (traceContext == null)
                 {
-                    // If parent is SpanContext but its TraceContext is null, then it was extracted from
-                    // propagation headers. Create a new TraceContext (this will start a new trace) and initialize
+                    var propagatedTags = parentSpanContext.PropagatedTags;
+                    var samplingPriority = parentSpanContext.SamplingPriority;
+
+                    // When in appsec standalone mode, only distributed traces with the `_dd.p.appsec` tag
+                    // are propagated downstream, however we need 1 trace per minute sent to the backend, so
+                    // we unset sampling priority so the rate limiter decides.
+                    if (Settings?.AppsecStandaloneEnabledInternal == true)
+                    {
+                        // If the trace has appsec propagation tag, the default priority is user keep
+                        samplingPriority = propagatedTags?.GetTag(Tags.Propagated.AppSec) == "1" ? SamplingPriorityValues.UserKeep : null;
+                    }
+
+                    // If parent is SpanContext but its TraceContext is null, then it was extracted from propagation headers.
+                    // Create a new TraceContext (this will start a new trace) and initialize
                     // it with the propagated values (sampling priority, origin, tags, W3C trace state, etc).
-                    traceContext = new TraceContext(this, parentSpanContext.PropagatedTags);
+                    traceContext = new TraceContext(this, propagatedTags);
                     TelemetryFactory.Metrics.RecordCountTraceSegmentCreated(MetricTags.TraceContinuation.Continued);
 
-                    var samplingPriority = parentSpanContext.SamplingPriority ?? DistributedTracer.Instance.GetSamplingPriority();
+                    samplingPriority ??= DistributedTracer.Instance.GetSamplingPriority();
                     traceContext.SetSamplingPriority(samplingPriority);
                     traceContext.Origin = parentSpanContext.Origin;
                     traceContext.AdditionalW3CTraceState = parentSpanContext.AdditionalW3CTraceState;