Add /health handler to node server

This commit adds a very simple /health handler that returns 200 if
the server is running. This allows to check whether the node server
is running.

This fixes restatedev#2461.

crates/node/src/network_server/metrics.rs

@@ -11,6 +11,7 @@
 use std::fmt::Write;
 
 use axum::extract::State;
+use http::StatusCode;
 use metrics_exporter_prometheus::formatting;
 use metrics_exporter_prometheus::{PrometheusBuilder, PrometheusHandle};
 use metrics_tracing_context::TracingContextLayer;
@@ -202,6 +203,10 @@ pub(crate) fn install_global_prometheus_recorder(opts: &CommonOptions) -> Promet
 }
 
 // -- Direct HTTP Handlers --
+pub async fn report_health() -> StatusCode {
+    StatusCode::OK
+}
+
 pub async fn render_metrics(State(state): State<NodeCtrlHandlerState>) -> String {
     let default_cf = CfName::new("default");
     let mut out = String::new();

crates/node/src/network_server/service.rs

@@ -15,7 +15,9 @@ use tokio::time::MissedTickBehavior;
 use tonic::codec::CompressionEncoding;
 use tracing::{debug, trace};
 
-use crate::network_server::metrics::{install_global_prometheus_recorder, render_metrics};
+use crate::network_server::metrics::{
+    install_global_prometheus_recorder, render_metrics, report_health,
+};
 use crate::network_server::state::NodeCtrlHandlerStateBuilder;
 use restate_core::network::protobuf::core_node_svc::core_node_svc_server::CoreNodeSvcServer;
 use restate_core::network::protobuf::node_ctl_svc::node_ctl_svc_server::NodeCtlSvcServer;
@@ -80,6 +82,7 @@ impl NetworkServer {
 
         // -- HTTP service (for prometheus et al.)
         let axum_router = axum::Router::new()
+            .route("/health", get(report_health))
             .route("/metrics", get(render_metrics))
             .route("/debug/pprof/heap", get(pprof::heap))
             .route(