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 https://github.com/prometheus-community/postgres_exporter/issues/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