RFE: add ability to prefix application_name to logs

[mikkorantalainen]

We're running pgbouncer 1.7 in production with high load and getting random failures with following in pgbouncer logs:

2020-04-20 08:51:42.513 2653 WARNING C-0x234e898: incoming-database/real-database@127.0.0.1:40974 Pooler Error: client_idle_timeout

(This particular message appears in the log after the connection between client and postgresql backend has hanged for long enough time - the actual issue is still under investigation and this issue is not about the actual error in example.)

It's really hard to correlate this message to any client transaction we have. The PostgreSQL server supports log_line_prefix (https://www.postgresql.org/docs/current/runtime-config-logging.html#GUC-LOG-LINE-PREFIX) which supports adding application_name to any log messages emitted.

Our client executes SET LOCAL application_name='unique-identifier-123' at the start of every transaction. We're using following setting for postgresql and it would be awesome if pgbouncer supported something similar in the future:

log_line_prefix='%m %u@%d/%p %a: '

Of course, pgbouncer has stuff that normal postgresql does not have such as potentially different database name on the incoming connection vs actual database name used with postgresql. As such, pgbouncer should support more escape sequences than postgresql itself. It would be nice to use as many escape sequences with the same meaning as possible. I think that if pgbouncer needs some escape sequences for stuff that postgresql does not have it might be good to reuse some of the sequences that pgbouncer does not need (e.g. %v or %x) so that there is not a risk for collision in future if/when postgresql introduces new sequences.