pgsql: instrumentation: Rename INSTR_TIME_LT macro to INSTR_TIME_GT

instrumentation: Rename INSTR_TIME_LT macro to INSTR_TIME_GT

This was incorrectly named "LT" for "larger than" in e5a5e0a90750d66, but
that is against existing conventions, where "LT" means "less than".
Clarify by using "GT" for "greater than" in macro name, and add a missing
comment at the top of instr_time.h to note the macro's existence.

Reported by: Peter Smith <smithpb2250(at)gmail(dot)com>
Author: Lukas Fittl <lukas(at)fittl(dot)com>
Reviewed-by: Andres Freund <andres(at)anarazel(dot)de>
Discussion: https://postgr.es/m/CAHut%2BPut94CTpjQsqOJHdHkgJ2ZXq%2BqVSfMEcmDKLiWLW-hPfA%40mail.gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/3218825271bd5ea3921ecd18e373bc76947c70bc

Modified Files
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src/backend/executor/instrument.c | 2 +-
src/include/portability/instr_time.h | 4 +++-
2 files changed, 4 insertions(+), 2 deletions(-)