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@paquier.xyz>
Reviewed-by: Chao Li <li.evan.chao@gmail.com>
Discussion: https://postgr.es/m/aMPKWR81KT5UXvEr@paquier.xyz