Do not set _dd.p.ksr when using the DEFAULT sampling mechanism

The ksr trace tag should only be set when the sampling decision comes
from an explicit source (agent rate, rule, or remote rule). When the
DEFAULT mechanism is used — meaning no agent configuration has been
received yet — the rate is a hardcoded 100% and ksr is meaningless.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: bm1549

src/datadog/trace_segment.cpp

@@ -328,8 +328,14 @@ void TraceSegment::make_sampling_decision_if_null() {
 
   update_decision_maker_trace_tag();
 
-  trace_tags_.emplace_back(tags::internal::ksr,
-                           format_rate(*sampling_decision_->configured_rate));
+  // Only set ksr when the sampling mechanism is explicit (agent rate, rule, or
+  // remote rule).  The DEFAULT mechanism means we haven't received any
+  // configuration from the agent yet, so ksr would be meaningless.
+  if (sampling_decision_->mechanism &&
+      *sampling_decision_->mechanism != int(SamplingMechanism::DEFAULT)) {
+    trace_tags_.emplace_back(tags::internal::ksr,
+                             format_rate(*sampling_decision_->configured_rate));
+  }
 }
 
 void TraceSegment::update_decision_maker_trace_tag() {

test/test_trace_segment.cpp

@@ -312,7 +312,9 @@ TEST_CASE("TraceSegment finalization of spans") {
       REQUIRE_THAT(span.tags, ContainsSubset(filtered));
       // "_dd.p.dm" will be added, because we made a sampling decision.
       REQUIRE(span.tags.count("_dd.p.dm") == 1);
-      REQUIRE(span.tags.count("_dd.p.ksr") == 1);
+      // "_dd.p.ksr" is NOT set because this uses the DEFAULT mechanism (no
+      // agent configuration received yet).
+      REQUIRE(span.tags.count("_dd.p.ksr") == 0);
     }
 
     SECTION("rate tags") {
@@ -327,7 +329,8 @@ TEST_CASE("TraceSegment finalization of spans") {
         REQUIRE(collector->span_count() == 1);
         const auto& span = collector->first_span();
         REQUIRE(span.numeric_tags.at(tags::internal::agent_sample_rate) == 1.0);
-        REQUIRE(span.tags.at(tags::internal::ksr) == "1");
+        // ksr is NOT set for the DEFAULT mechanism.
+        REQUIRE(span.tags.count(tags::internal::ksr) == 0);
       }
 
       SECTION(
@@ -350,7 +353,8 @@ TEST_CASE("TraceSegment finalization of spans") {
         {
           REQUIRE(collector_response->span_count() == 1);
           const auto& span = collector_response->first_span();
-          CHECK(span.tags.at(tags::internal::ksr) == "1");
+          // ksr is NOT set for the DEFAULT mechanism.
+          CHECK(span.tags.count(tags::internal::ksr) == 0);
         }
 
         collector_response->chunks.clear();