Add pg_stat_wal table metrics

[pandar00]

• Adds the following metrics. pg_stat_wal table. It is off by default

"wal_records", "wal_fpi", // bigint "wal_bytes", // numeric "wal_buffers_full", // bigint "wal_write", // bigint "wal_sync", // bigint "wal_write_time", // double precision "wal_sync_time", // double precision "stats_reset", // timestamp with time zone 

• Mostly copied from pg_stat_bgwriter metric implementation because it is essentially the same except for the column names.
• Resolves Add support for wal_fpi in pg_stat_wal #1028

# HELP pg_stat_wal_stats_reset Time at which these statistics were last reset # TYPE pg_stat_wal_stats_reset counter pg_stat_wal_stats_reset 1.730575462e+09 # HELP pg_stat_wal_wal_buffers_full Number of times WAL data was written to disk because WAL buffers became full # TYPE pg_stat_wal_wal_buffers_full counter pg_stat_wal_wal_buffers_full 1.040989e+06 # HELP pg_stat_wal_wal_bytes Total amount of WAL generated in bytes # TYPE pg_stat_wal_wal_bytes counter pg_stat_wal_wal_bytes 8.8115219412e+10 # HELP pg_stat_wal_wal_fpi Total number of WAL full page images generated # TYPE pg_stat_wal_wal_fpi counter pg_stat_wal_wal_fpi 1.2850296e+07 # HELP pg_stat_wal_wal_records Total number of WAL records generated # TYPE pg_stat_wal_wal_records counter pg_stat_wal_wal_records 3.3412574e+07 # HELP pg_stat_wal_wal_sync Number of times WAL files were synced to disk via issue_xlog_fsync request (if fsync is on and wal_sync_method is either fdatasync, fsync or fsync_writethrough, otherwise zero). See Section 30.5 for more information about the internal WAL function issue_xlog_fsync. # TYPE pg_stat_wal_wal_sync counter pg_stat_wal_wal_sync 74409 # HELP pg_stat_wal_wal_sync_time Total amount of time spent syncing WAL files to disk via issue_xlog_fsync request,	in milliseconds (if track_wal_io_timing is enabled, fsync is on, and wal_sync_method is either fdatasync, fsync or fsync_writethrough, otherwise zero). # TYPE pg_stat_wal_wal_sync_time counter pg_stat_wal_wal_sync_time 0 # HELP pg_stat_wal_wal_write Number of times WAL buffers were written out to disk via XLogWrite request. See Section 30.5 for more information about the internal WAL function XLogWrite. # TYPE pg_stat_wal_wal_write counter pg_stat_wal_wal_write 1.128863e+06 # HELP pg_stat_wal_wal_write_time Total amount of time spent writing WAL buffers to disk via XLogWrite request, in milliseconds (if track_wal_io_timing is enabled, otherwise zero). This includes the sync time when wal_sync_method is either open_datasync or open_sync. # TYPE pg_stat_wal_wal_write_time counter pg_stat_wal_wal_write_time 0 

[sysadmind]

Bump @SuperQ