Allow sampling of statements depending on duration
 
This allows logging a sample of statements, without incurring excessive
log traffic (which may impact performance).  This can be useful when
analyzing workloads with lots of short queries.
 
The sampling is configured using two new GUC parameters:
 
 * log_min_duration_sample - minimum required statement duration
 
 * log_statement_sample_rate - sample rate (0.0 - 1.0)
 
Only statements with duration exceeding log_min_duration_sample are
considered for sampling. To enable sampling, both those GUCs have to
be set correctly.
 
The existing log_min_duration_statement GUC has a higher priority, i.e.
statements with duration exceeding log_min_duration_statement will be
always logged, irrespectedly of how the sampling is configured. This
means only configurations
 
  log_min_duration_sample < log_min_duration_statement
 
do actually sample the statements, instead of logging everything.
 
Author: Adrien Nayrat
 
Discussion: https://postgr.es/m/-a8f7-3be2-155a-@anayrat.info