refactor: move get_or_init into TracerSignature 3-param constructor

The 3-param constructor now calls root_session_id::get_or_init internally,
so every TracerSignature construction via the short form goes through the
singleton. Tracer no longer needs to call get_or_init directly.

Session header tests use the 4-param constructor to control values
directly without depending on singleton state.

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

Author: khanayan123

BUILD.bazel

@@ -84,6 +84,7 @@ cc_library(
         "src/datadog/trace_source.cpp",
         "src/datadog/tracer.cpp",
         "src/datadog/tracer_config.cpp",
+        "src/datadog/tracer_signature.cpp",
         "src/datadog/version.cpp",
         "src/datadog/w3c_propagation.cpp",
         "src/datadog/w3c_propagation.h",

CMakeLists.txt

@@ -212,6 +212,7 @@ target_sources(dd-trace-cpp-objects
     src/datadog/tag_propagation.cpp
     src/datadog/threaded_event_scheduler.cpp
     src/datadog/tracer_config.cpp
+    src/datadog/tracer_signature.cpp
     src/datadog/tracer.cpp
     src/datadog/trace_id.cpp
     src/datadog/trace_sampler_config.cpp

include/datadog/tracer_signature.h

@@ -42,10 +42,7 @@ struct TracerSignature {
 
   TracerSignature() = delete;
   TracerSignature(RuntimeID runtime_id, std::string default_service,
-                  std::string default_environment)
-      : TracerSignature(runtime_id, runtime_id.string(),
-                        std::move(default_service),
-                        std::move(default_environment)) {}
+                  std::string default_environment);
   TracerSignature(RuntimeID runtime_id, std::string root_session_id,
                   std::string default_service, std::string default_environment)
       : runtime_id(runtime_id),

src/datadog/tracer.cpp

@@ -24,7 +24,6 @@
 #include "msgpack.h"
 #include "platform_util.h"
 #include "random.h"
-#include "root_session_id.h"
 #include "span_data.h"
 #include "span_sampler.h"
 #include "tags.h"
@@ -49,9 +48,8 @@ Tracer::Tracer(const FinalizedTracerConfig& config,
     : logger_(config.logger),
       runtime_id_(config.runtime_id ? *config.runtime_id
                                     : RuntimeID::generate()),
-      signature_{runtime_id_,
-                 root_session_id::get_or_init(runtime_id_.string()),
-                 config.defaults.service, config.defaults.environment},
+      signature_{runtime_id_, config.defaults.service,
+                 config.defaults.environment},
       config_manager_(std::make_shared<ConfigManager>(config)),
       collector_(/* see constructor body */),
       span_sampler_(

src/datadog/tracer_signature.cpp

@@ -0,0 +1,17 @@
+#include <datadog/tracer_signature.h>
+
+#include "root_session_id.h"
+
+namespace datadog {
+namespace tracing {
+
+TracerSignature::TracerSignature(RuntimeID runtime_id,
+                                 std::string default_service,
+                                 std::string default_environment)
+    : TracerSignature(runtime_id,
+                      root_session_id::get_or_init(runtime_id.string()),
+                      std::move(default_service),
+                      std::move(default_environment)) {}
+
+}  // namespace tracing
+}  // namespace datadog

test/telemetry/test_telemetry.cpp

@@ -374,7 +374,8 @@ TELEMETRY_IMPLEMENTATION_TEST("session ID headers") {
 
   SECTION("root process: DD-Session-ID present, DD-Root-Session-ID absent") {
     auto session_rid = RuntimeID::generate();
-    const TracerSignature sig{session_rid, "testsvc", "test"};
+    const TracerSignature sig{session_rid, session_rid.string(), "testsvc",
+                              "test"};
 
     Telemetry telemetry{*finalize_config(), sig, logger, client,
                         scheduler,          *url};