feat(sidecar): wire telemetry_extended_heartbeat_interval through SessionConfig (#1882)

## Summary

Add `telemetry_extended_heartbeat_interval` to the sidecar's `SessionConfig` and FFI
layer so that callers (e.g. PHP tracer) can configure the extended heartbeat interval.

The telemetry worker already supports this field in its `Config` struct, but the sidecar
was not passing it through from `SessionConfig` -> `modify_telemetry_config`. This meant
sidecar consumers could not control the cadence of `app-extended-heartbeat` events.

**Changes:**
- `datadog-sidecar/src/service/mod.rs`: add `telemetry_extended_heartbeat_interval: Duration` to `SessionConfig`
- `datadog-sidecar/src/service/sidecar_server.rs`: set `cfg.telemetry_extended_heartbeat_interval` in `modify_telemetry_config`
- `datadog-sidecar-ffi/src/lib.rs`: add `telemetry_extended_heartbeat_interval_millis: u64` param to `ddog_sidecar_session_set_config`

Companion dd-trace-php PR: DataDog/dd-trace-php#3800

Co-authored-by: bwoebi <bob.weinand@datadoghq.com>

Author: khanayan123

datadog-sidecar-ffi/src/lib.rs

@@ -614,6 +614,7 @@ pub unsafe extern "C" fn ddog_sidecar_session_set_config(
     flush_interval_milliseconds: u32,
     remote_config_poll_interval_millis: u32,
     telemetry_heartbeat_interval_millis: u32,
+    telemetry_extended_heartbeat_interval_millis: u64,
     force_flush_size: usize,
     force_drop_size: usize,
     log_level: ffi::CharSlice,
@@ -643,6 +644,9 @@ pub unsafe extern "C" fn ddog_sidecar_session_set_config(
         telemetry_heartbeat_interval: Duration::from_millis(
             telemetry_heartbeat_interval_millis as u64,
         ),
+        telemetry_extended_heartbeat_interval: Duration::from_millis(
+            telemetry_extended_heartbeat_interval_millis,
+        ),
         force_flush_size,
         force_drop_size,
         log_level: log_level.to_utf8_lossy().into(),

datadog-sidecar-ffi/tests/sidecar.rs

@@ -98,6 +98,7 @@ fn test_ddog_sidecar_register_app() {
             1000,
             1000000,
             1,
+            86400000,
             10000000,
             10000000,
             "".into(),
@@ -151,6 +152,7 @@ fn test_ddog_sidecar_register_app() {
             1000,
             1000000,
             1,
+            86400000,
             10000000,
             10000000,
             "".into(),

datadog-sidecar/src/service/mod.rs

@@ -58,6 +58,7 @@ pub struct SessionConfig {
     pub flush_interval: Duration,
     pub remote_config_poll_interval: Duration,
     pub telemetry_heartbeat_interval: Duration,
+    pub telemetry_extended_heartbeat_interval: Duration,
     pub force_flush_size: usize,
     pub force_drop_size: usize,
     pub log_level: String,

datadog-sidecar/src/service/sidecar_server.rs

@@ -640,12 +640,16 @@ impl SidecarInterface for ConnectionSidecarHandler {
         *session.process_tags.lock_or_panic() = config.process_tags.clone();
         session.modify_telemetry_config(|cfg| {
             cfg.telemetry_heartbeat_interval = config.telemetry_heartbeat_interval;
+            cfg.telemetry_extended_heartbeat_interval =
+                config.telemetry_extended_heartbeat_interval;
             let endpoint = get_product_endpoint(
                 libdd_telemetry::config::PROD_INTAKE_SUBDOMAIN,
                 &config.endpoint,
             );
             cfg.set_endpoint(endpoint).ok();
             cfg.telemetry_heartbeat_interval = config.telemetry_heartbeat_interval;
+            cfg.telemetry_extended_heartbeat_interval =
+                config.telemetry_extended_heartbeat_interval;
         });
         session.modify_trace_config(|cfg| {
             let endpoint = get_product_endpoint(

libdd-telemetry/src/worker/mod.rs

@@ -483,9 +483,9 @@ impl TelemetryWorker {
                 self.data.integrations.unflush_stored();
                 self.data.configurations.unflush_stored();
 
-                let app_started = data::Payload::AppStarted(self.build_app_started());
-                match self.send_payload(&app_started).await {
-                    Ok(()) => self.payload_sent_success(&app_started),
+                let extended_hb = data::Payload::AppExtendedHeartbeat(self.build_app_started());
+                match self.send_payload(&extended_hb).await {
+                    Ok(()) => self.payload_sent_success(&extended_hb),
                     Err(err) => self.log_err(&err),
                 }
                 #[allow(clippy::unwrap_used)]