fix(telemetry): wire up DD_TELEMETRY_EXTENDED_HEARTBEAT_INTERVAL to scheduler (#1824)

## Summary
Wire up the `DD_TELEMETRY_EXTENDED_HEARTBEAT_INTERVAL` config value to the telemetry scheduler. The env var was already parsed into `config.telemetry_extended_heartbeat_interval` but the scheduler hardcoded `Duration::from_secs(60 * 60 * 24)` instead of using it.

Default remains 24h this only enables system tests to use a shorter interval to validate the `app-extended-heartbeat` event fires correctly.

## Changes
- **`libdd-telemetry/src/worker/mod.rs`**: Replace hardcoded `60 * 60 * 24` with `config.telemetry_extended_heartbeat_interval`

## Motivation
Cross-SDK system tests need to set a short extended heartbeat interval (e.g., 2s) to validate parity of the `app-extended-heartbeat` telemetry event across all SDKs. Without this fix, PHP and other libdatadog consumers cannot be system-tested for this feature.

## Related
- System test PR: DataDog/system-tests#6338

Co-authored-by: edmund.kump <edmund.kump@datadoghq.com>

Author: khanayan123

libdd-telemetry/Cargo.toml

@@ -45,3 +45,4 @@ winver = "1.0.0"
 [dev-dependencies]
 tracing-subscriber = "0.3.22"
 tokio = { version = "1.23", features = ["sync", "io-util", "rt-multi-thread"] }
+libdd-common = { path = "../libdd-common", features = ["test-utils"] }

libdd-telemetry/src/config.rs

@@ -27,6 +27,7 @@ pub struct Config {
     /// Enables debug logging
     pub telemetry_debug_logging_enabled: bool,
     pub telemetry_heartbeat_interval: Duration,
+    pub telemetry_extended_heartbeat_interval: Duration,
     pub direct_submission_enabled: bool,
     /// Prevents LifecycleAction::Stop from terminating the worker (except if the WorkerHandle is
     /// dropped)
@@ -163,6 +164,7 @@ impl Default for Config {
             endpoint: None,
             telemetry_debug_logging_enabled: false,
             telemetry_heartbeat_interval: Duration::from_secs(60),
+            telemetry_extended_heartbeat_interval: Duration::from_secs(60 * 60 * 24),
             direct_submission_enabled: false,
             restartable: false,
             debug_enabled: false,
@@ -246,6 +248,7 @@ impl Config {
             endpoint: None,
             telemetry_debug_logging_enabled: settings.shared_lib_debug,
             telemetry_heartbeat_interval: settings.telemetry_heartbeat_interval,
+            telemetry_extended_heartbeat_interval: settings.telemetry_extended_heartbeat_interval,
             direct_submission_enabled: settings.direct_submission_enabled,
             restartable: false,
             debug_enabled: false,
@@ -473,4 +476,42 @@ mod tests {
                 .to_string_lossy()
         );
     }
+
+    #[test]
+    fn test_from_settings_propagates_extended_heartbeat_interval() {
+        use std::time::Duration;
+
+        let custom_interval = Duration::from_secs(120);
+        let settings = Settings {
+            telemetry_extended_heartbeat_interval: custom_interval,
+            ..Default::default()
+        };
+        let cfg = Config::from_settings(&settings);
+        assert_eq!(cfg.telemetry_extended_heartbeat_interval, custom_interval);
+    }
+
+    #[test]
+    fn test_from_settings_default_extended_heartbeat_interval() {
+        use std::time::Duration;
+
+        let settings = Settings::default();
+        let cfg = Config::from_settings(&settings);
+        assert_eq!(
+            cfg.telemetry_extended_heartbeat_interval,
+            Duration::from_secs(60 * 60 * 24)
+        );
+    }
+
+    #[test]
+    fn test_extended_heartbeat_interval_from_env() {
+        use libdd_common::test_utils::EnvGuard;
+        use std::time::Duration;
+
+        let _guard = EnvGuard::set("DD_TELEMETRY_EXTENDED_HEARTBEAT_INTERVAL", "5");
+        let settings = Settings::from_env();
+        assert_eq!(
+            settings.telemetry_extended_heartbeat_interval,
+            Duration::from_secs(5)
+        );
+    }
 }

libdd-telemetry/src/worker/mod.rs

@@ -1108,6 +1108,7 @@ impl TelemetryWorkerBuilder {
         let token = CancellationToken::new();
         let config = self.config;
         let telemetry_heartbeat_interval = config.telemetry_heartbeat_interval;
+        let telemetry_extended_heartbeat_interval = config.telemetry_extended_heartbeat_interval;
         let client = http_client::from_config(&config);
 
         let metrics_flush_interval =
@@ -1140,7 +1141,7 @@ impl TelemetryWorkerBuilder {
                 (metrics_flush_interval, LifecycleAction::FlushMetricAggr),
                 (telemetry_heartbeat_interval, LifecycleAction::FlushData),
                 (
-                    time::Duration::from_secs(60 * 60 * 24),
+                    telemetry_extended_heartbeat_interval,
                     LifecycleAction::ExtendedHeartbeat,
                 ),
             ]),