Register root session ID env var in environment registry

Use environment::lookup() instead of std::getenv() directly, and
register DD_ROOT_CPP_SESSION_ID in DD_LIST_ENVIRONMENT_VARIABLES
per project convention.

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

Author: khanayan123

include/datadog/environment.h

@@ -81,7 +81,11 @@ namespace environment {
   MACRO(DD_APM_TRACING_ENABLED, BOOLEAN, true)                                 \
   MACRO(DD_TRACE_RESOURCE_RENAMING_ENABLED, BOOLEAN, false)                    \
   MACRO(DD_TRACE_RESOURCE_RENAMING_ALWAYS_SIMPLIFIED_ENDPOINT, BOOLEAN, false) \
-  MACRO(DD_EXTERNAL_ENV, STRING, "")
+  MACRO(DD_EXTERNAL_ENV, STRING, "")                                           \
+  MACRO(DD_ROOT_CPP_SESSION_ID, STRING,                                        \
+        ENV_DEFAULT_RESOLVED_IN_CODE(                                          \
+            "Internal: root session ID inherited across exec. Set "            \
+            "automatically by the tracer; do not set manually."))
 
 #define ENV_DEFAULT_RESOLVED_IN_CODE(X) X
 #define WITH_COMMA(ARG, TYPE, DEFAULT_VALUE) ARG,
@@ -95,7 +99,7 @@ enum Variable { DD_LIST_ENVIRONMENT_VARIABLES(WITH_COMMA) };
 #define QUOTED_WITH_COMMA(ARG, TYPE, DEFAULT_VALUE) \
   WITH_COMMA(QUOTED(ARG), TYPE, DEFAULT_VALUE)
 
-inline const char *const variable_names[] = {
+inline const char* const variable_names[] = {
     DD_LIST_ENVIRONMENT_VARIABLES(QUOTED_WITH_COMMA)};
 
 #undef QUOTED

src/datadog/tracer.cpp

@@ -40,8 +40,8 @@ namespace tracing {
 namespace {
 
 std::string get_root_session_id(const std::string& runtime_id) {
-  if (const char* env = std::getenv("_DD_ROOT_CPP_SESSION_ID")) {
-    return std::string(env);
+  if (auto val = environment::lookup(environment::DD_ROOT_CPP_SESSION_ID)) {
+    return std::string(*val);
   }
   return runtime_id;
 }
@@ -76,7 +76,8 @@ Tracer::Tracer(const FinalizedTracerConfig& config,
       baggage_extraction_enabled_(false),
       tracing_enabled_(config.tracing_enabled),
       resource_renaming_mode_(config.resource_renaming_mode) {
-  setenv("_DD_ROOT_CPP_SESSION_ID", signature_.root_session_id.c_str(), 1);
+  setenv(environment::name(environment::DD_ROOT_CPP_SESSION_ID).data(),
+         signature_.root_session_id.c_str(), 1);
 
   telemetry::init(config.telemetry, signature_, logger_, config.http_client,
                   config.event_scheduler, config.agent_url);