Re: Add statistics to pg_stat_wal view for wal related parameter tuning

I think it's really a more convenient way to collect wal usage information,
with it we can query when I want. Several points on my side:

1. It will be nice If you provide a chance to reset the information in WalStats,
so that we can reset it without restart the database.

2. I think 'wal_write_backend' is mean wal write times happen in
backends. The describe in document is not so clear, suggest rewrite it.

3. I do not think it's a correct describe in document for 'wal_buffers_full'.

4. Quite strange to collect twice in XLogInsertRecord() for xl_tot_len,
m_wal_records, m_wal_fpi.

5. I notice some code in issue_xlog_fsync() function to collect sync info,
a standby may call the issue_xlog_fsync() too, which do not want to
to collect this info. I think this need some change avoid run by standby
side.

Regards,
Highgo Software (Canada/China/Pakistan)
URL : www.highgo.ca
EMAIL: mailto:movead(dot)li(at)highgo(dot)ca