fix: snapshot entity metrics on main thread

src/main/java/com/prometheus/spigot/PrometheusSpigotPlugin.java

@@ -17,7 +17,9 @@ import java.net.InetSocketAddress;
 import java.util.Locale;
 import java.util.concurrent.ExecutorService;
 import java.util.concurrent.Executors;
+import org.bukkit.Bukkit;
 import org.bukkit.plugin.java.JavaPlugin;
+import org.bukkit.scheduler.BukkitTask;
 
 public final class PrometheusSpigotPlugin extends JavaPlugin {
     private HttpServer httpServer;
@@ -25,6 +27,7 @@ public final class PrometheusSpigotPlugin extends JavaPlugin {
     private MetricsRegistry metricsRegistry;
     private String bearerToken;
     private boolean countBlockMovementOnly;
+    private BukkitTask snapshotTask;
 
     @Override
     public void onEnable() {
@@ -43,6 +46,8 @@ public final class PrometheusSpigotPlugin extends JavaPlugin {
 
         metricsRegistry = new MetricsRegistry(getServer(), enableJvm, enableProcess);
 
+        scheduleSnapshotTask();
+
         registerListeners();
         startHttpServer();
     }
@@ -57,6 +62,10 @@ public final class PrometheusSpigotPlugin extends JavaPlugin {
             httpExecutor.shutdownNow();
             httpExecutor = null;
         }
+        if (snapshotTask != null) {
+            snapshotTask.cancel();
+            snapshotTask = null;
+        }
     }
 
     private void registerListeners() {
@@ -95,4 +104,12 @@ public final class PrometheusSpigotPlugin extends JavaPlugin {
         httpServer.start();
         getLogger().info(String.format(Locale.ROOT, "Prometheus endpoint listening on http://%s:%d%s", bindAddress, port, path));
     }
+
+    private void scheduleSnapshotTask() {
+        int intervalSeconds = getConfig().getInt("metrics.entity_snapshot_interval_seconds", 15);
+        long intervalTicks = Math.max(20L, intervalSeconds * 20L);
+
+        metricsRegistry.refreshEntitySnapshots();
+        snapshotTask = Bukkit.getScheduler().runTaskTimer(this, metricsRegistry::refreshEntitySnapshots, intervalTicks, intervalTicks);
+    }
 }