pgsql: injection_points: Enable entry count in its variable-sized stats

injection_points: Enable entry count in its variable-sized stats

This serves as coverage for the tracking of entry count added by
7bd2975fa92b as built-in variable-sized stats kinds have no need for it,
at least not yet.

A new function, called injection_points_stats_count(), is added to the
module. It is able to return the number of entries. This has been
useful when doing some benchmarking to check the sanity of the counts.

Author: Michael Paquier <michael(at)paquier(dot)xyz>
Reviewed-by: Chao Li <li(dot)evan(dot)chao(at)gmail(dot)com>
Discussion: https://postgr.es/m/aMPKWR81KT5UXvEr@paquier.xyz

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/acf0960c23090ea6690f2c5da3bc2625836516a9

Modified Files
--------------
src/test/modules/injection_points/injection_points--1.0.sql | 10 ++++++++++
src/test/modules/injection_points/injection_stats.c | 12 ++++++++++++
src/test/modules/injection_points/t/001_stats.pl | 12 ++++++++++++
3 files changed, 34 insertions(+)