Fix supply AdminClient creation (kafbat#875)

Author: germanosin

api/src/main/java/io/kafbat/ui/service/AdminClientServiceImpl.java

@@ -16,6 +16,7 @@
 import org.apache.kafka.clients.admin.AdminClientConfig;
 import org.springframework.stereotype.Service;
 import reactor.core.publisher.Mono;
+import reactor.core.scheduler.Schedulers;
 
 @Service
 @Slf4j
@@ -41,7 +42,7 @@ public Mono<ReactiveAdminClient> get(KafkaCluster cluster) {
   }
 
   private Mono<ReactiveAdminClient> createAdminClient(KafkaCluster cluster) {
-    return Mono.fromFuture(CompletableFuture.supplyAsync(() -> {
+    return Mono.fromSupplier(() -> {
       Properties properties = new Properties();
       KafkaClientSslPropertiesUtil.addKafkaSslProperties(cluster.getOriginalProperties().getSsl(), properties);
       properties.putAll(cluster.getProperties());
@@ -52,7 +53,8 @@ private Mono<ReactiveAdminClient> createAdminClient(KafkaCluster cluster) {
           "kafbat-ui-admin-" + Instant.now().getEpochSecond() + "-" + CLIENT_ID_SEQ.incrementAndGet()
       );
       return AdminClient.create(properties);
-    })).flatMap(ac -> ReactiveAdminClient.create(ac).doOnError(th -> ac.close()))
+    }).subscribeOn(Schedulers.boundedElastic())
+        .flatMap(ac -> ReactiveAdminClient.create(ac).doOnError(th -> ac.close()))
         .onErrorMap(th -> new IllegalStateException(
             "Error while creating AdminClient for the cluster " + cluster.getName(), th));
   }