If the stats collector dies during Hot Standby, restart it.

This bug exists as far back as 9.0, when Hot Standby was introduced,
so back-patch to all supported branches.

Report and patch by Takayuki Tsunakawa, reviewed by Michael Paquier
and Kuntal Ghosh.

src/backend/postmaster/postmaster.c

@@ -1753,7 +1753,8 @@ ServerLoop(void)
 		}
 
 		/* If we have lost the stats collector, try to start a new one */
-		if (PgStatPID == 0 && pmState == PM_RUN)
+		if (PgStatPID == 0 &&
+			(pmState == PM_RUN || pmState == PM_HOT_STANDBY))
 			PgStatPID = pgstat_start();
 
 		/* If we have lost the archiver, try to start a new one. */
@@ -2963,7 +2964,7 @@ reaper(SIGNAL_ARGS)
 			if (!EXIT_STATUS_0(exitstatus))
 				LogChildExit(LOG, _("statistics collector process"),
 							 pid, exitstatus);
-			if (pmState == PM_RUN)
+			if (pmState == PM_RUN || pmState == PM_HOT_STANDBY)
 				PgStatPID = pgstat_start();
 			continue;
 		}