issues/1506: ensure Prometheus /metrics endpoint streams correctly wi…

api/src/main/java/io/kafbat/ui/controller/PrometheusExposeController.java

@@ -8,6 +8,7 @@
 import java.util.Optional;
 import java.util.stream.Collectors;
 import lombok.RequiredArgsConstructor;
+import org.springframework.http.MediaType;
 import org.springframework.http.ResponseEntity;
 import org.springframework.web.bind.annotation.RestController;
 import org.springframework.web.server.ServerWebExchange;
@@ -21,27 +22,45 @@ public class PrometheusExposeController extends AbstractController implements Pr
 
   @Override
   public Mono<ResponseEntity<String>> exposeAllMetrics(ServerWebExchange exchange) {
+
+    String metrics = PrometheusMetricsExposer.exposeAllMetrics(
+        clustersStorage.getKafkaClusters().stream()
+            .filter(KafkaCluster::isExposeMetricsViaPrometheusEndpoint)
+            .collect(Collectors.toMap(
+                KafkaCluster::getName,
+                c -> statisticsCache.get(c).getMetrics()
+            ))
+    ).getBody();
+
     return Mono.just(
-        PrometheusMetricsExposer.exposeAllMetrics(
-            clustersStorage.getKafkaClusters()
-                .stream()
-                .filter(KafkaCluster::isExposeMetricsViaPrometheusEndpoint)
-                .collect(Collectors.toMap(KafkaCluster::getName, c -> statisticsCache.get(c).getMetrics()))
-        )
+        ResponseEntity
+            .ok()
+            .contentType(MediaType.parseMediaType("text/plain; version=0.0.4"))
+            .body(metrics)
     );
   }
 
   @Override
-  public Mono<ResponseEntity<String>> exposeClusterMetrics(String clusterName,
-                                                           ServerWebExchange exchange) {
+  public Mono<ResponseEntity<String>> exposeClusterMetrics(String clusterName, ServerWebExchange exchange) {
+
     Optional<KafkaCluster> cluster = clustersStorage.getClusterByName(clusterName);
-    if (cluster.isPresent() && cluster.get().isExposeMetricsViaPrometheusEndpoint()) {
-      return Mono.just(PrometheusMetricsExposer.exposeAllMetrics(
-          Map.of(clusterName, statisticsCache.get(cluster.get()).getMetrics())
-      ));
-    } else {
+
+    if (cluster.isEmpty() || !cluster.get().isExposeMetricsViaPrometheusEndpoint()) {
       return Mono.just(ResponseEntity.notFound().build());
     }
-  }
 
+    String metrics = PrometheusMetricsExposer.exposeAllMetrics(
+        Map.of(
+            clusterName,
+            statisticsCache.get(cluster.get()).getMetrics()
+        )
+    ).getBody();
+
+    return Mono.just(
+        ResponseEntity
+            .ok()
+            .contentType(MediaType.parseMediaType("text/plain; version=0.0.4"))
+            .body(metrics)
+    );
+  }
 }