feat: seed root session ID in master for stable telemetry headers (#345)

chore: update dd-trace-cpp submodule to stable session ID headers branch

Points to ayan.khan/stable-session-id-headers tip for integration
validation of DD-Session-ID and DD-Root-Session-ID telemetry headers.

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

feat: seed root session ID in master before workers fork

Generate root_session_id in create_datadog_main_conf (master process)
and pass it to TracerConfig in make_tracer (worker process). All workers
inherit the same root via fork memory copy, ensuring consistent
DD-Root-Session-ID across telemetry from all workers.

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

chore: update dd-trace-cpp submodule to latest

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

fix: remove submodule pin, keep only src changes

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

Merge branch 'master' into ayan.khan/stable-session-id-headers

chore: update dd-trace-cpp submodule to main (includes #295)

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

Merge branch 'master' into ayan.khan/stable-session-id-headers

Co-authored-by: ayan.khan <ayan.khan@datadoghq.com>

Author: khanayan123

dd-trace-cpp

@@ -94,6 +94,7 @@ struct datadog_main_conf_t {
   // module configuration, instead we grab the values from the environment
   // of the master process and apply them later in the worker processes after
   // `fork()`.
+  std::string root_session_id;
   std::vector<environment_variable_t> environment_variables;
   // If `propagation_styles` is empty, then use the defaults instead.
   // `propagation_styles` is populated by the "datadog_propagation_styles"

src/datadog_conf.h

@@ -1,5 +1,7 @@
 #include "ngx_http_datadog_module.h"
 
+#include <datadog/runtime_id.h>
+
 #include <cassert>
 #include <cstdlib>
 #include <exception>
@@ -408,6 +410,7 @@ static void *create_datadog_main_conf(ngx_conf_t *conf) noexcept {
   if (register_destructor(conf->pool, main_conf)) {
     return nullptr;  // error
   }
+  main_conf->root_session_id = dd::RuntimeID::generate().string();
   return main_conf;
 }
 

src/ngx_http_datadog_module.cpp

@@ -55,6 +55,10 @@ dd::Expected<dd::Tracer> TracingLibrary::make_tracer(
   config.integration_version = NGINX_VERSION;
   config.service = "nginx";
 
+  if (!nginx_conf.root_session_id.empty()) {
+    config.root_session_id = nginx_conf.root_session_id;
+  }
+
   if (nginx_conf.apm_tracing_enabled != NGX_CONF_UNSET) {
     config.tracing_enabled = {nginx_conf.apm_tracing_enabled == 1};
   }