pgsql: Add log_statement_sample_rate parameter

Add log_statement_sample_rate parameter

This allows to set a lower log_min_duration_statement value without
incurring excessive log traffic (which reduces performance).  This can
be useful to analyze workloads with lots of short queries.

Author: Adrien Nayrat
Reviewed-by: David Rowley, Vik Fearing
Discussion: https://postgr.es/m/c30ee535-ee1e-db9f-fa97-146b9f62caed@anayrat.info

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/88bdbd3f746049834ae3cc972e6e650586ec3c9d

Modified Files
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doc/src/sgml/config.sgml                      | 30 ++++++++++++++++++++++-----
src/backend/tcop/postgres.c                   | 16 ++++++++++++--
src/backend/utils/misc/guc.c                  | 15 +++++++++++++-
src/backend/utils/misc/postgresql.conf.sample | 13 +++++++-----
src/include/utils/guc.h                       |  1 +
5 files changed, 62 insertions(+), 13 deletions(-)