feat: re-add root_session_id to TracerConfig for integration seeding

Integrations (nginx, httpd, kong) need to set root_session_id in the
master process before workers fork. The Tracer passes it to get_or_init,
falling back to runtime_id if not provided.

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

Author: khanayan123

include/datadog/tracer_config.h

@@ -189,6 +189,11 @@ struct TracerConfig {
   // This option is ignored if `resource_renaming_enabled` is not `true`.
   Optional<bool> resource_renaming_always_simplified_endpoint;
 
+  // Root session ID for stable telemetry correlation across forked workers.
+  // Integrations (nginx, httpd, kong) should set this in the master process
+  // before workers fork so all Tracers share the same root.
+  Optional<std::string> root_session_id;
+
   /// A mapping of process-specific tags used to uniquely identify processes.
   ///
   /// The `process_tags` map allows associating arbitrary string-based keys and
@@ -225,6 +230,7 @@ class FinalizedTracerConfig final {
   bool log_on_startup;
   bool generate_128bit_trace_ids;
   Optional<RuntimeID> runtime_id;
+  Optional<std::string> root_session_id;
   Clock clock;
   std::string integration_name;
   std::string integration_version;

src/datadog/tracer.cpp

@@ -50,7 +50,8 @@ Tracer::Tracer(const FinalizedTracerConfig& config,
       runtime_id_(config.runtime_id ? *config.runtime_id
                                     : RuntimeID::generate()),
       signature_{runtime_id_,
-                 root_session_id::get_or_init(runtime_id_.string()),
+                 root_session_id::get_or_init(
+                     config.root_session_id.value_or(runtime_id_.string())),
                  config.defaults.service, config.defaults.environment},
       config_manager_(std::make_shared<ConfigManager>(config)),
       collector_(/* see constructor body */),

src/datadog/tracer_config.cpp

@@ -407,6 +407,10 @@ Expected<FinalizedTracerConfig> finalize_config(const TracerConfig &user_config,
     final_config.runtime_id = user_config.runtime_id;
   }
 
+  if (user_config.root_session_id) {
+    final_config.root_session_id = user_config.root_session_id;
+  }
+
   final_config.process_tags = user_config.process_tags;
 
   auto agent_finalized =