Fix _dd.p.ksr formatting to use 6 significant digits without trailing zeros

Replace std::to_string() (which uses sprintf "%f" producing 6 trailing
decimal places) with snprintf "%.6g" which produces up to 6 significant
digits with no trailing zeros. This matches the behavior of Python's
f"{rate:.6g}" and Go's strconv.FormatFloat(rate, 'g', 6, 64).

Examples: 1.0 -> "1", 0.5 -> "0.5", 0.0 -> "0"

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: bm1549

src/datadog/trace_segment.cpp

@@ -13,6 +13,7 @@
 
 #include <cassert>
 #include <charconv>
+#include <cstdio>
 #include <string>
 #include <unordered_map>
 #include <utility>
@@ -34,6 +35,12 @@ namespace datadog {
 namespace tracing {
 namespace {
 
+std::string format_rate(double rate) {
+  char buf[32];
+  std::snprintf(buf, sizeof(buf), "%.6g", rate);
+  return std::string(buf);
+}
+
 struct Cache {
   static int process_id;
 
@@ -323,7 +330,7 @@ void TraceSegment::make_sampling_decision_if_null() {
 
   trace_tags_.emplace_back(
       tags::internal::ksr,
-      std::to_string(*sampling_decision_->configured_rate));
+      format_rate(*sampling_decision_->configured_rate));
 }
 
 void TraceSegment::update_decision_maker_trace_tag() {

test/test_span.cpp

@@ -784,8 +784,8 @@ TEST_SPAN("injecting W3C tracestate header") {
            {"x-datadog-parent-id", "1"},
            {"x-datadog-origin", "France"},
        },
-       // The "s:-1" and "t.ksr:0.000000" comes from the 0% sample rate.
-       "dd=s:-1;p:$parent_id;o:France;t.ksr:0.000000"},
+       // The "s:-1" and "t.ksr:0" comes from the 0% sample rate.
+       "dd=s:-1;p:$parent_id;o:France;t.ksr:0"},
 
       {__LINE__,
        "trace tags",
@@ -794,8 +794,8 @@ TEST_SPAN("injecting W3C tracestate header") {
            {"x-datadog-parent-id", "1"},
            {"x-datadog-tags", "_dd.p.foo=x,_dd.p.bar=y,ignored=wrong_prefix"},
        },
-       // The "s:-1" and "t.ksr:0.000000"  comes from the 0% sample rate.
-       "dd=s:-1;p:$parent_id;t.foo:x;t.bar:y;t.ksr:0.000000"},
+       // The "s:-1" and "t.ksr:0"  comes from the 0% sample rate.
+       "dd=s:-1;p:$parent_id;t.foo:x;t.bar:y;t.ksr:0"},
 
       {__LINE__,
        "extra fields",
@@ -825,7 +825,7 @@ TEST_SPAN("injecting W3C tracestate header") {
           },
           // The "s:-1" comes from the 0% sample rate.
           "dd=s:-1;p:$parent_id;o:France_ is a country~nation_ so is "
-          "______.;t.ksr:0.000000",
+          "______.;t.ksr:0",
       },
 
       {__LINE__,
@@ -836,7 +836,7 @@ TEST_SPAN("injecting W3C tracestate header") {
            {"x-datadog-tags", "_dd.p.a;d台北x =foo,_dd.p.ok=bar"},
        },
        // The "s:-1" comes from the 0% sample rate.
-       "dd=s:-1;p:$parent_id;t.a_d______x_:foo;t.ok:bar;t.ksr:0.000000"},
+       "dd=s:-1;p:$parent_id;t.a_d______x_:foo;t.ok:bar;t.ksr:0"},
 
       {__LINE__,
        "replace invalid characters in trace tag value",
@@ -847,7 +847,7 @@ TEST_SPAN("injecting W3C tracestate header") {
        },
        // The "s:-1" comes from the 0% sample rate.
        "dd=s:-1;p:$parent_id;t.wacky:hello fr_d_ how are "
-       "_________?;t.ksr:0.000000"},
+       "_________?;t.ksr:0"},
 
       {__LINE__,
        "replace equal signs with tildes in trace tag value",
@@ -857,7 +857,7 @@ TEST_SPAN("injecting W3C tracestate header") {
            {"x-datadog-tags", "_dd.p.base64_thingy=d2Fra2EhIHdhaw=="},
        },
        // The "s:-1" comes from the 0% sample rate.
-       "dd=s:-1;p:$parent_id;t.base64_thingy:d2Fra2EhIHdhaw~~;t.ksr:0.000000"},
+       "dd=s:-1;p:$parent_id;t.base64_thingy:d2Fra2EhIHdhaw~~;t.ksr:0"},
 
       {__LINE__,
        "oversized origin truncates it and subsequent fields",

test/test_trace_segment.cpp

@@ -7,7 +7,9 @@
 #include <datadog/tracer.h>
 #include <datadog/tracer_config.h>
 
+#include <cstdio>
 #include <regex>
+#include <string>
 #include <vector>
 
 #include "matchers.h"
@@ -325,7 +327,7 @@ 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) == "1.000000");
+        REQUIRE(span.tags.at(tags::internal::ksr) == "1");
       }
 
       SECTION(
@@ -348,7 +350,7 @@ 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) == "1.000000");
+          CHECK(span.tags.at(tags::internal::ksr) == "1");
         }
 
         collector_response->chunks.clear();
@@ -361,7 +363,7 @@ TEST_CASE("TraceSegment finalization of spans") {
           REQUIRE(collector_response->span_count() == 1);
           const auto& span = collector_response->first_span();
           CHECK(span.numeric_tags.at(tags::internal::agent_sample_rate) == 1.0);
-          CHECK(span.tags.at(tags::internal::ksr) == "1.000000");
+          CHECK(span.tags.at(tags::internal::ksr) == "1");
         }
       }
 
@@ -392,7 +394,11 @@ TEST_CASE("TraceSegment finalization of spans") {
         const auto& span = collector->first_span();
         REQUIRE(span.numeric_tags.at(tags::internal::rule_sample_rate) ==
                 sample_rate);
-        CHECK(span.tags.at(tags::internal::ksr) == std::to_string(sample_rate));
+        {
+          char buf[32];
+          std::snprintf(buf, sizeof(buf), "%.6g", sample_rate);
+          CHECK(span.tags.at(tags::internal::ksr) == std::string(buf));
+        }
         if (sample_rate == 1.0) {
           REQUIRE(span.numeric_tags.at(
                       tags::internal::rule_limiter_sample_rate) == 1.0);