logfmt and application_context

Hi everyone,

I just release a logfmt log collector for PostgreSQL :
https://pgxn.org/dist/logfmt/1.0.0/ . This works quite well but I have
a few issues I would like to share with hackers.

First, what do you think of having logfmt output along json and CSV ?
PostgreSQL internal syslogger has builtin support for the different
LOG_DESTINATION_*. Thus logfmt does not send log collector headers
using write_syslogger_file or write_pipe_chunks but plain log line with
write_console. Do you have some hint about this ? The consequences ?
How much is it a good bet to write a custom log collector in a shared
preload library ?

Second issue, logfmt provides a guc called
`logfmt.application_context`. The purpose of application_context is the
same as `application_name` but for a more varying value like request
UUID, task ID, etc. What do you think of this ? Would it be cool to
have this GUC in PostgreSQL and available in log_line_prefix ?

Anyway, it's my first attempt at writing C code for PostgreSQL, with
the help of Guillaume LELARGE and Jehan-Guillaume de RORTHAIS and it's
a pleasure ! PostgreSQL C code is very readable. Thanks everyone for
this !

Regards,
Étienne BERSAC
Developer at Dalibo