refactor: move root session ID singleton internals to .cpp file

Hide instance() and flag() in an anonymous namespace in
root_session_id.cpp, matching the telemetry Meyer's singleton pattern.
Only set() and get() are exposed in the header.

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

Author: khanayan123

BUILD.bazel

@@ -45,6 +45,7 @@ cc_library(
         "src/datadog/propagation_style.cpp",
         "src/datadog/random.cpp",
         "src/datadog/random.h",
+        "src/datadog/root_session_id.cpp",
         "src/datadog/root_session_id.h",
         "src/datadog/rate.cpp",
         "src/datadog/remote_config/product.cpp",

CMakeLists.txt

@@ -197,6 +197,7 @@ target_sources(dd-trace-cpp-objects
     src/datadog/parse_util.cpp
     src/datadog/propagation_style.cpp
     src/datadog/random.cpp
+    src/datadog/root_session_id.cpp
     src/datadog/rate.cpp
     src/datadog/remote_config/product.cpp
     src/datadog/remote_config/remote_config.cpp

src/datadog/root_session_id.cpp

@@ -0,0 +1,31 @@
+#include "root_session_id.h"
+
+#include <mutex>
+
+namespace datadog {
+namespace tracing {
+namespace root_session_id {
+
+namespace {
+
+std::string& instance() {
+  static std::string id;
+  return id;
+}
+
+std::once_flag& flag() {
+  static std::once_flag f;
+  return f;
+}
+
+}  // namespace
+
+void set(const std::string& id) {
+  std::call_once(flag(), [&]() { instance() = id; });
+}
+
+const std::string& get() { return instance(); }
+
+}  // namespace root_session_id
+}  // namespace tracing
+}  // namespace datadog

src/datadog/root_session_id.h

@@ -1,34 +1,17 @@
 #pragma once
 
-#include <mutex>
 #include <string>
 
 namespace datadog {
 namespace tracing {
+namespace root_session_id {
 
 // Thread-safe singleton for the root session ID. The first call to `set`
 // captures the value; subsequent calls are no-ops. Immutable after init,
 // so fork-safe.
-
-namespace root_session_id {
-
-inline std::string& instance() {
-  static std::string id;
-  return id;
-}
-
-inline std::once_flag& flag() {
-  static std::once_flag f;
-  return f;
-}
-
-inline void set(const std::string& id) {
-  std::call_once(flag(), [&]() { instance() = id; });
-}
-
-inline const std::string& get() { return instance(); }
+void set(const std::string& id);
+const std::string& get();
 
 }  // namespace root_session_id
-
 }  // namespace tracing
 }  // namespace datadog