Log all statements from a sample of transactions

This is useful to obtain a view of the different transaction types in an
application, regardless of the durations of the statements each runs.

Author: Adrien Nayrat
Reviewed-by: Masahiko Sawada, Hayato Kuroda, Andres Freund

src/backend/utils/misc/postgresql.conf.sample

@@ -495,6 +495,10 @@
 					# log_min_duration_statement. 1.0 logs all statements,
 					# 0 never logs.
 
+#log_transaction_sample_rate = 0.0	# Fraction of transactions whose statements
+					# are logged regardless of their duration. 1.0 logs all
+					# statements from all transactions, 0.0 never logs.
+
 # - What to Log -
 
 #debug_print_parse = off