fix: _dd.p.ksr formatting to use 6 significant digits without trailing zeros (#288)

* 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>

* Fix clang-format violation in trace_segment.cpp

Reformat the emplace_back call to match clang-format's expected style
(arguments on one line with alignment) to fix the verify CI job.

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

* 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>

* Replace snprintf with std::to_chars for _dd.p.ksr formatting

Address PR feedback: remove format_rate function and use
std::to_chars with std::chars_format::general inline instead
of snprintf.

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

* Update src/datadog/trace_segment.cpp

Co-authored-by: Damien Mehala <damien.mehala@datadoghq.com>

* Apply suggestion from @bm1549

* Fix clang-format violation and logic bug in ksr error handling

Fix two issues introduced by GitHub suggestion commits:
1. Correct the error condition: `ec == std::errc()` means success, not
   failure — changed to `ec != std::errc()` to properly detect errors.
2. Fix indentation from 4-space to 2-space to match project style and
   pass clang-format checks.

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

---------

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>
Co-authored-by: Damien Mehala <damien.mehala@datadoghq.com>

Author: 3 people

src/datadog/trace_segment.cpp

@@ -11,6 +11,7 @@
 #include <datadog/telemetry/telemetry.h>
 #include <datadog/trace_segment.h>
 
+#include <array>
 #include <cassert>
 #include <charconv>
 #include <string>
@@ -321,9 +322,23 @@ void TraceSegment::make_sampling_decision_if_null() {
 
   update_decision_maker_trace_tag();
 
-  trace_tags_.emplace_back(
-      tags::internal::ksr,
-      std::to_string(*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)) {
+    std::array<char, 8> buf;
+    const auto [ptr, ec] = std::to_chars(buf.data(), buf.data() + buf.size(),
+                                         *sampling_decision_->configured_rate,
+                                         std::chars_format::general, 6);
+    if (ec != std::errc()) {
+      std::string error{"string conversion failed: "};
+      error += std::make_error_code(ec).message();
+      logger_->log_error(error);
+      return;
+    }
+    trace_tags_.emplace_back(tags::internal::ksr, std::string(buf.data(), ptr));
+  }
 }
 
 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"
@@ -310,7 +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") {
@@ -325,7 +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) == "1.000000");
+        // ksr is NOT set for the DEFAULT mechanism.
+        REQUIRE(span.tags.count(tags::internal::ksr) == 0);
       }
 
       SECTION(
@@ -348,7 +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) == "1.000000");
+          // ksr is NOT set for the DEFAULT mechanism.
+          CHECK(span.tags.count(tags::internal::ksr) == 0);
         }
 
         collector_response->chunks.clear();
@@ -361,7 +367,7 @@ 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 +398,11 @@ 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);