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

Dear Ikeda-san,

Thank you for updating the patch. This can be applied to master, and
can be used on my RHEL7.
wal_write_time and wal_sync_time increase normally :-).

```
postgres=# select * from pg_stat_wal;
-[ RECORD 1 ]----+------------------------------
wal_records      | 121781
wal_fpi          | 2287
wal_bytes        | 36055146
wal_buffers_full | 799
wal_write        | 12770
wal_write_time   | 4.469
wal_sync         | 11962
wal_sync_time    | 132.352
stats_reset      | 2021-01-25 00:51:40.674412+00
```

I put a further comment:

```
@@ -3485,7 +3485,53 @@ SELECT pid, wait_event_type, wait_event FROM pg_stat_activity WHERE wait_event i
        wal_buffers_full bigint
       
       
-       Number of times WAL data was written to disk because WAL buffers became full
+       Total number of times WAL data was written to disk because WAL buffers became full
+      
+     
+
+     
+      
+       wal_write bigint
+      
+      
+       Total number of times WAL data was written to disk
+      
+     
+
+     
+      
+       wal_write_time double precision
+      
+      
+       Total amount of time that has been spent in the portion of
+       WAL data was written to disk, in milliseconds
+       (if  is enabled, otherwise zero).
+      
+     
+
+     
+      
+       wal_sync bigint
+      
+      
+       Total number of times WAL data was synced to disk
+       (if  is open_datasync or 
+       open_sync, this value is zero because WAL data is synced 
+       when to write it).
+      
+     
+
+     
+      
+       wal_sync_time double precision
+      
+      
+       Total amount of time that has been spent in the portion of
+       WAL data was synced to disk, in milliseconds
+       (if  is enabled, otherwise zero.
+       if  is open_datasync or 
+       open_sync, this value is zero too because WAL data is synced 
+       when to write it).
       
      
 ```

Maybe "Total amount of time" should be used, not "Total number of time."
Other views use "amount."

I have no comments anymore.

Hayato Kuroda
FUJITSU LIMITED