Tracelog truncation can make output longer.

This is a pretty small nitpick, but I noticed:

https://github.com/jackc/pgx/blob/ea9610f6722e0c6c6073660f14ded3c9683a593e/tracelog/tracelog.go#L101

The message indicating truncation can take up more space than the number of bytes being removed - i.e. if you commonly have values of 65 bytes, it will truncate 1 byte but print out an extra ` (truncated 1 bytes)`, which is 20 more bytes. Maybe changing this can be rolled into a solution for https://github.com/jackc/pgx/issues/998

Start at tracelog/tracelog.go around line 101 and inspect how truncation length and the truncation message are calculated. Review issue #998 for the related direction, then verify that truncating values does not produce output longer than the original value.

Written by the indexing model from the issue text.