Add wal_fpi_bytes to wal_stats metric

[nadashaban11]

Description

After reading https://postgr.es/c/f9a09aa29 I added wal_fpi_bytes to wal_stats metric
Fixes #1399

I've added wal_fpi_bytes to the wal_stats metric by adding a new SQL query block for version 19 (duplicating the version 18 query and adding the new column).

Also, from reading the commit, I noticed that wal_fpi_bytes is also available on pg_stat_get_backend_wal(), but pgwatch doesn't currently have backend-specific WAL stats metric, I didn't create a new metric for it.

I am opening this as a draft to make sure I correctly understood what's needed.

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Checklist

• Code compiles and existing tests pass locally.
• New or updated tests are included where applicable.
• Documentation is updated where applicable.