process type escape for log_line_prefix

Hi,

Frequently when reading postgres logs to do some post mortem analysis
I'm left wondering what process emitted an error/log message. After the
fact it's often hard to know wether an error message was emitted by a
user backend or by something internal, say the checkpointer or
autovacuum. Logging all process starts is often impractical given the
log volume that causes.

So I'm proposing adding an escape displaying the process title (say 'k'
for kind?). So %k would emit something like "autovacuum worker process",
"wal sender process" etc.

I'm thinking it might make sense to give normal connections "" as the
name, they're usually already discernible.

Greetings,

Andres Freund