Add prometheus metrics, currently only meteor_discord_users_total

src/main/java/org/meteordev/meteorbot/MeteorBot.java

@@ -37,7 +37,7 @@ public static void main(String[] args) {
         JDABuilder.createDefault(token)
             .enableIntents(GatewayIntent.GUILD_MEMBERS, GatewayIntent.MESSAGE_CONTENT)
             .enableCache(CacheFlag.EMOJI)
-            .addEventListeners(new MeteorBot(), new Commands(), new Uptime(), new InfoChannels())
+            .addEventListeners(new MeteorBot(), new Commands(), new Uptime(), new InfoChannels(), new Metrics())
             .build();
     }
 

src/main/java/org/meteordev/meteorbot/Metrics.java

@@ -0,0 +1,78 @@
+package org.meteordev.meteorbot;
+
+import com.sun.net.httpserver.HttpExchange;
+import com.sun.net.httpserver.HttpServer;
+import net.dv8tion.jda.api.JDA;
+import net.dv8tion.jda.api.entities.Guild;
+import net.dv8tion.jda.api.events.session.ReadyEvent;
+import net.dv8tion.jda.api.events.session.ShutdownEvent;
+import net.dv8tion.jda.api.hooks.ListenerAdapter;
+
+import java.io.IOException;
+import java.io.OutputStream;
+import java.net.InetSocketAddress;
+import java.nio.charset.StandardCharsets;
+
+public class Metrics extends ListenerAdapter {
+    private Guild guild;
+    private HttpServer server;
+
+    @Override
+    public void onReady(ReadyEvent event) {
+        JDA bot = event.getJDA();
+
+        guild = bot.getGuildById(Env.GUILD_ID.value);
+        if (guild == null) {
+            MeteorBot.LOG.error("Couldn't find the specified server.");
+            System.exit(1);
+        }
+
+        try {
+            server = HttpServer.create(new InetSocketAddress(9400), 0);
+            server.createContext("/metrics", this::onRequest);
+            server.start();
+        } catch (IOException e) {
+            throw new RuntimeException(e);
+        }
+
+        MeteorBot.LOG.info("Providing metrics on :9400/metrics");
+    }
+
+    @Override
+    public void onShutdown(ShutdownEvent event) {
+        server.stop(1000);
+    }
+
+    private void onRequest(HttpExchange exchange) {
+        byte[] response = String.format("""
+            # HELP meteor_discord_users_total Total number of Discord users in our server
+            # TYPE meteor_discord_users_total gauge
+            meteor_discord_users_total %d
+            """, guild.getMemberCount()).getBytes(StandardCharsets.UTF_8);
+
+        try {
+            exchange.getResponseHeaders().set("Content-Type", "text/plain");
+            exchange.sendResponseHeaders(200, response.length);
+        } catch (IOException e) {
+            throw new RuntimeException(e);
+        }
+
+        OutputStream out = exchange.getResponseBody();
+        copy(response, out);
+    }
+
+    @SuppressWarnings("ThrowFromFinallyBlock")
+    private static void copy(byte[] in, OutputStream out) {
+        try {
+            out.write(in);
+        } catch (IOException e) {
+            throw new RuntimeException(e);
+        } finally {
+            try {
+                out.close();
+            } catch (IOException e) {
+                throw new RuntimeException(e);
+            }
+        }
+    }
+}