Add a write_to_file member to PgStat_KindInfo

Hi hackers,

Working on [1] produced the need to give to the statistics the ability to
decide whether or not they want to be written to the file on disk.

Indeed, there is no need to write the per backend I/O stats to disk (no point to
see stats for backends that do not exist anymore after a re-start), so $SUBJECT.

This new member could also be useful for custom statistics, so creating this
dedicated thread.

The attached patch also adds a test in the fixed injection points statistics.
It does not add one for the variable injection points statistics as I think adding
one test is enough (the code changes are simple enough).

[1]: https://www.postgresql.org/message-id/Zy4bmvgHqGjcK1pI%40ip-10-97-1-34.eu-west-3.compute.internal

Looking forward to your feedback,

Regards,

-- 
Bertrand Drouvot
PostgreSQL Contributors Team
RDS Open Source Databases
Amazon Web Services: https://aws.amazon.com