Identify walsenders in pg_stat_activity

Following 8299471c walsender procs are now visible in pg_stat_activity.
Set query to ‘walsender’ for walsender procs to allow them to be identified.

Discussion:CAB7nPqS8c76KPSufK_HSDeYrbtg+zZ7D0EEkjeM6txSEuCB_jA@mail.gmail.com

Michael Paquier, issue raised by Fujii Masao, reviewed by Tom Lane

src/backend/replication/walsender.c

@@ -1806,6 +1806,9 @@ WalSndLoop(WalSndSendDataCallback send_data)
 	last_reply_timestamp = GetCurrentTimestamp();
 	waiting_for_ping_response = false;
 
+	/* Report to pgstat that this process is a WAL sender */
+	pgstat_report_activity(STATE_RUNNING, "walsender");
+
 	/*
 	 * Loop until we reach the end of this timeline or the client requests to
 	 * stop streaming.