Add information about database pool to metrics endpoint in streaming API (#23469)

streaming/index.js

@@ -856,6 +856,15 @@ const startWorker = async (workerId) => {
     res.write('# TYPE connected_channels gauge\n');
     res.write('# HELP connected_channels The number of Redis channels the streaming server is subscribed to\n');
     res.write(`connected_channels ${Object.keys(subs).length}.0\n`);
+    res.write('# TYPE pg.pool.total_connections gauge \n');
+    res.write('# HELP pg.pool.total_connections The total number of clients existing within the pool\n');
+    res.write(`pg.pool.total_connections ${pgPool.totalCount}.0\n`);
+    res.write('# TYPE pg.pool.idle_connections gauge \n');
+    res.write('# HELP pg.pool.idle_connections The number of clients which are not checked out but are currently idle in the pool\n');
+    res.write(`pg.pool.idle_connections ${pgPool.idleCount}.0\n`);
+    res.write('# TYPE pg.pool.waiting_queries gauge \n');
+    res.write('# HELP pg.pool.waiting_queries The number of queued requests waiting on a client when all clients are checked out\n');
+    res.write(`pg.pool.waiting_queries ${pgPool.waitingCount}.0\n`);
     res.write('# EOF\n');
     res.end();
   }));