[#28638] YSQL: Add additional columns to the rpcz endpoint

Summary:
This diff adds additional columns pid and pss_mem_bytes to rpcz endpoint and corrects the logic for capturing leader_pid column. These columns are pid, pss_mm_bytes and leader_pid. The leader_pid field was already being captured internally in the yb_pg_metrics.c through the PostgreSQL lock group leader mechanism,  but was failing to identify parallel workers. The new implementation identifies parallel workers by retrieving the PGPROC structure for backend processes and background workers.
pid: Process ID of the process.
pss_mem_bytes: PSS memory usage in bytes for the process. It returns -1 if unavailable or tracking is disabled.
leader_pid: Leader PID if this is a parallel worker. This will be -1 for leaders and non-parallel backends.

Test Plan:
Output at http://localhost:13000/rpcz

{
    "connections": [
        {
            "process_start_time": "2025-11-04 13:39:01.277151+05:30",
            "application_name": "yb_ash collector",
            "backend_type": "background worker",
            "backend_status": "",
            "pid": 71979,
            "pss_mem_bytes": -1
        },
        {
            "db_oid": 16384,
            "db_name": "cdb",
            "query": "EXPLAIN ANALYSE SELECT COUNT(*) FROM test;",
            "query_id": 16749132132008374320,
            "leader_pid": 72084,
            "process_start_time": "2025-11-04 13:48:19.052474+05:30",
            "process_running_for_ms": 1722,
            "transaction_start_time": "2025-11-04 13:48:18.922652+05:30",
            "transaction_running_for_ms": 1852,
            "query_start_time": "2025-11-04 13:48:18.922652+05:30",
            "query_running_for_ms": 1852,
            "application_name": "ysqlsh",
            "backend_type": "background worker",
            "backend_status": "active",
            "pid": 72510,
            "pss_mem_bytes": -1
        },
        {
            "db_oid": 16384,
            "db_name": "cdb",
            "query": "EXPLAIN ANALYSE SELECT COUNT(*) FROM test;",
            "query_id": 16749132132008374320,
            "process_start_time": "2025-11-04 13:39:44.18782+05:30",
            "process_running_for_ms": 516587,
            "transaction_start_time": "2025-11-04 13:48:18.922652+05:30",
            "transaction_running_for_ms": 1852,
            "query_start_time": "2025-11-04 13:48:18.922652+05:30",
            "query_running_for_ms": 1852,
            "application_name": "ysqlsh",
            "backend_type": "client backend",
            "backend_status": "active",
            "host": "127.0.0.1",
            "port": "57558",
            "pid": 72084,
            "pss_mem_bytes": -1
        },
        {
            "db_oid": 16384,
            "db_name": "cdb",
            "query": "EXPLAIN ANALYSE SELECT COUNT(*) FROM test;",
            "query_id": 16749132132008374320,
            "leader_pid": 72084,
            "process_start_time": "2025-11-04 13:48:19.05245+05:30",
            "process_running_for_ms": 1722,
            "transaction_start_time": "2025-11-04 13:48:18.922652+05:30",
            "transaction_running_for_ms": 1852,
            "query_start_time": "2025-11-04 13:48:18.922652+05:30",
            "query_running_for_ms": 1852,
            "application_name": "ysqlsh",
            "backend_type": "background worker",
            "backend_status": "active",
            "pid": 72509,
            "pss_mem_bytes": -1
        },
        {
            "process_start_time": "2025-11-04 13:39:01.271293+05:30",
            "application_name": "",
            "backend_type": "checkpointer",
            "backend_status": "",
            "pid": 71976,
            "pss_mem_bytes": -1
        }
    ]
}

Reviewers: asaha, ishan.chhangani, hbhanawat, cagrawal

Reviewed By: asaha

Subscribers: svc_phabricator, yql

Differential Revision: https://phorge.dev.yugabyte.com/D47924

Author: shobhitomkumar

src/postgres/yb-extensions/yb_pg_metrics/yb_pg_metrics.c

@@ -590,6 +590,7 @@ pullRpczEntries(void)
 			PGPROC	   *proc = NULL;
 
 			if (beentry->st_backendType == B_BACKEND ||
+				beentry->st_backendType == B_BG_WORKER ||
 				beentry->st_backendType == YB_AUTO_ANALYZE_BACKEND)
 				proc = BackendPidGetProc(rpcz[i].proc_id);
 			else if (beentry->st_backendType != YB_YSQL_CONN_MGR)
@@ -605,7 +606,11 @@ pullRpczEntries(void)
 			{
 				PGPROC	   *leader = proc->lockGroupLeader;
 
-				if (leader != NULL)
+				/*
+				 * Show the leader only for active parallel workers.  This leaves
+				 * the field as -1 (NULL equivalent) for the leader of a parallel group.
+				 */
+				if (leader != NULL && leader->pid != beentry->st_procpid)
 				{
 					rpcz[i].leader_pid = leader->pid;
 				}
@@ -680,6 +685,11 @@ pullRpczEntries(void)
 				rpcz[i].host = NULL;
 				rpcz[i].port = NULL;
 			}
+
+			int64_t rss_bytes, pss_bytes;
+			YbPgGetCurRssPssMemUsage(rpcz[i].proc_id, &rss_bytes, &pss_bytes);
+			rpcz[i].pss_mem_bytes = pss_bytes;
+
 			after_changecount = beentry->st_changecount;
 
 			if (before_changecount == after_changecount &&

src/yb/yql/pggate/webserver/ybc_pg_webserver_wrapper.cc

@@ -455,6 +455,12 @@ static void PgRpczHandler(const Webserver::WebRequest &req, Webserver::WebRespon
         writer.String(entry->port);
       }
 
+      writer.String("pid");
+      writer.Int(entry->proc_id);
+
+      writer.String("pss_mem_bytes");
+      writer.Int64(entry->pss_mem_bytes);
+
       writer.EndObject();
     }
   }

src/yb/yql/pggate/webserver/ybc_pg_webserver_wrapper.h

@@ -64,6 +64,7 @@ typedef struct YbcRpczEntry {
   char *backend_status;
   char *host;
   char *port;
+  int64 pss_mem_bytes;
 } YbcRpczEntry;
 
 typedef struct {