feat: Add server registered/unregistered events (#1386)

* feat: Add server registered/unregistered events

* Annotate new API with `@Beta`

* Migrate from classes to records

* Add null checks

* Fix code style indent

* Add links in documentation

* Fix docs indent

---------

Co-authored-by: powercas_gamer <cas@mizule.dev>

api/src/main/java/com/velocitypowered/api/event/proxy/server/ServerRegisteredEvent.java

@@ -0,0 +1,31 @@
+/*
+ * Copyright (C) 2024 Velocity Contributors
+ *
+ * The Velocity API is licensed under the terms of the MIT License. For more details,
+ * reference the LICENSE file in the api top-level directory.
+ */
+
+package com.velocitypowered.api.event.proxy.server;
+
+import com.google.common.annotations.Beta;
+import com.google.common.base.Preconditions;
+import com.velocitypowered.api.proxy.server.RegisteredServer;
+import com.velocitypowered.api.proxy.server.ServerInfo;
+import org.jetbrains.annotations.NotNull;
+
+/**
+ * This event is fired by the proxy after a backend server is registered to the server map.
+ * Currently, it may occur when a server is registered dynamically at runtime or when a server is
+ * replaced due to configuration reload.
+ *
+ * @see com.velocitypowered.api.proxy.ProxyServer#registerServer(ServerInfo)
+ *
+ * @param registeredServer A {@link RegisteredServer} that has been registered.
+ * @since 3.3.0
+ */
+@Beta
+public record ServerRegisteredEvent(@NotNull RegisteredServer registeredServer) {
+  public ServerRegisteredEvent {
+    Preconditions.checkNotNull(registeredServer, "registeredServer");
+  }
+}

api/src/main/java/com/velocitypowered/api/event/proxy/server/ServerUnregisteredEvent.java

@@ -0,0 +1,31 @@
+/*
+ * Copyright (C) 2024 Velocity Contributors
+ *
+ * The Velocity API is licensed under the terms of the MIT License. For more details,
+ * reference the LICENSE file in the api top-level directory.
+ */
+
+package com.velocitypowered.api.event.proxy.server;
+
+import com.google.common.annotations.Beta;
+import com.google.common.base.Preconditions;
+import com.velocitypowered.api.proxy.server.RegisteredServer;
+import com.velocitypowered.api.proxy.server.ServerInfo;
+import org.jetbrains.annotations.NotNull;
+
+/**
+ * This event is fired by the proxy after a backend server is unregistered from the server map.
+ * Currently, it may occur when a server is unregistered dynamically at runtime
+ * or when a server is replaced due to configuration reload.
+ *
+ * @see com.velocitypowered.api.proxy.ProxyServer#unregisterServer(ServerInfo)
+ *
+ * @param unregisteredServer A {@link RegisteredServer} that has been unregistered.
+ * @since 3.3.0
+ */
+@Beta
+public record ServerUnregisteredEvent(@NotNull RegisteredServer unregisteredServer) {
+  public ServerUnregisteredEvent {
+    Preconditions.checkNotNull(unregisteredServer, "unregisteredServer");
+  }
+}

proxy/src/main/java/com/velocitypowered/proxy/server/ServerMap.java

@@ -19,6 +19,8 @@ package com.velocitypowered.proxy.server;
 
 import com.google.common.base.Preconditions;
 import com.google.common.collect.ImmutableList;
+import com.velocitypowered.api.event.proxy.server.ServerRegisteredEvent;
+import com.velocitypowered.api.event.proxy.server.ServerUnregisteredEvent;
 import com.velocitypowered.api.proxy.server.RegisteredServer;
 import com.velocitypowered.api.proxy.server.ServerInfo;
 import com.velocitypowered.proxy.VelocityServer;
@@ -84,6 +86,10 @@ public class ServerMap {
       throw new IllegalArgumentException(
           "Server with name " + serverInfo.getName() + " already registered");
     } else if (existing == null) {
+      if (server != null) {
+        server.getEventManager().fireAndForget(new ServerRegisteredEvent(rs));
+      }
+
       return rs;
     } else {
       return existing;
@@ -107,5 +113,9 @@ public class ServerMap {
         "Trying to remove server %s with differing information", serverInfo.getName());
     Preconditions.checkState(servers.remove(lowerName, rs),
         "Server with name %s replaced whilst unregistering", serverInfo.getName());
+
+    if (server != null) {
+      server.getEventManager().fireAndForget(new ServerUnregisteredEvent(rs));
+    }
   }
 }