feat: add DD_TRACE_PROPAGATION_BEHAVIOR_EXTRACT config

Adds the new `DD_TRACE_PROPAGATION_BEHAVIOR_EXTRACT` config key (values:
continue, restart, ignore) with a CUSTOM(INT) parser, the corresponding
C enum, and the supported-configurations.json entry.

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

Author: MilanGarnier

metadata/supported-configurations.json

@@ -1995,6 +1995,13 @@
         "default": "false"
       }
     ],
+    "DD_TRACE_PROPAGATION_BEHAVIOR_EXTRACT": [
+      {
+        "implementation": "A",
+        "type": "string",
+        "default": "continue"
+      }
+    ],
     "DD_TRACE_PROPAGATION_STYLE": [
       {
         "implementation": "D",

tracer/configuration.h

@@ -99,6 +99,7 @@
     CONFIG(SET_LOWERCASE, DD_TRACE_PROPAGATION_STYLE, "datadog,tracecontext,baggage",                          \
            .env_config_fallback = ddtrace_conf_otel_propagators)                                               \
     CONFIG(SET, DD_TRACE_BAGGAGE_TAG_KEYS, "user.id, session.id, account.id")                                  \
+    CONFIG(CUSTOM(INT), DD_TRACE_PROPAGATION_BEHAVIOR_EXTRACT, "continue", .parser = dd_parse_propagation_behavior_extract) \
     CONFIG(BOOL, DD_TRACE_IGNORE_AGENT_SAMPLING_RATES, "false", .ini_change = zai_config_system_ini_change)    \
     CONFIG(SET, DD_TRACE_TRACED_INTERNAL_FUNCTIONS, "")                                                        \
     CONFIG(INT, DD_TRACE_DEBUG_PRNG_SEED, "-1", .ini_change = ddtrace_reseed_seed_change)                      \
@@ -192,6 +193,12 @@ enum ddtrace_sampling_rules_format {
     DD_TRACE_SAMPLING_RULES_FORMAT_GLOB
 };
 
+enum ddtrace_propagation_behavior_extract {
+    DD_TRACE_PROPAGATION_BEHAVIOR_EXTRACT_CONTINUE = 0,
+    DD_TRACE_PROPAGATION_BEHAVIOR_EXTRACT_RESTART,
+    DD_TRACE_PROPAGATION_BEHAVIOR_EXTRACT_IGNORE,
+};
+
 #define DD_CONFIGURATION DDTRACE_CONFIGURATION
 #include <ext/configuration_helpers.h>
 

tracer/configuration_dependencies.h

@@ -22,6 +22,21 @@ static bool dd_parse_dbm_mode(zai_str value, zval *decoded_value, bool persisten
     return true;
 }
 
+static bool dd_parse_propagation_behavior_extract(zai_str value, zval *decoded_value, bool persistent) {
+    UNUSED(persistent);
+    if (zai_str_eq_ci_cstr(value, "continue")) {
+        ZVAL_LONG(decoded_value, DD_TRACE_PROPAGATION_BEHAVIOR_EXTRACT_CONTINUE);
+    } else if (zai_str_eq_ci_cstr(value, "restart")) {
+        ZVAL_LONG(decoded_value, DD_TRACE_PROPAGATION_BEHAVIOR_EXTRACT_RESTART);
+    } else if (zai_str_eq_ci_cstr(value, "ignore")) {
+        ZVAL_LONG(decoded_value, DD_TRACE_PROPAGATION_BEHAVIOR_EXTRACT_IGNORE);
+    } else {
+        return false;
+    }
+
+    return true;
+}
+
 static bool dd_parse_sampling_rules_format(zai_str value, zval *decoded_value, bool persistent) {
     UNUSED(persistent);
     if (zai_str_eq_ci_cstr(value, "regex")) {