RE: About to add WAL write/fsync statistics to pg_stat_wal view

Dear Ikeda-san,

This patch cannot be applied to the HEAD, but anyway I put a comment.

```
+    /*
+     * Measure i/o timing to fsync WAL data.
+     *
+     * The wal receiver skip to collect it to avoid performance degradation of standy servers.
+     * If sync_method doesn't have its fsync method, to skip too.
+     */
+    if (!AmWalReceiverProcess() && track_wal_io_timing && fsyncMethodCalled())
+        INSTR_TIME_SET_CURRENT(start);
```

I think m_wal_sync_time should be collected even if the process is WalRecevier.
Because all wal_fsync should be recorded, and
some performance issues have been aleady occurred if track_wal_io_timing is turned on.
I think it's strange only to take care of the walrecevier case.

Best Regards,
Hayato Kuroda
FUJITSU LIMITED