injection_points: Add some cumulative stats for injection points

This acts as a template of what can be achieved with the pluggable
cumulative stats APIs introduced in 7949d95945 for the
variable-numbered case where stats entries are stored in the pgstats
dshash, while being potentially useful on its own for injection points,
say to add starting and/or stopping conditions based on the statistics
(want to trigger a callback after N calls, for example?).

Currently, the only data gathered is the number of times an injection
point is run.  More fields can always be added as required.  All the
routines related to the stats are located in their own file, called
injection_stats.c in the test module injection_points, for clarity.

The stats can be used only if the test module is loaded through
shared_preload_libraries.  The key of the dshash uses InvalidOid for the
database, and an int4 hash of the injection point name as object ID.

A TAP test is added to provide coverage for the new custom cumulative
stats APIs, showing the persistency of the data across restarts, for
example.

Author: Michael Paquier
Reviewed-by: Dmitry Dolgov, Bertrand Drouvot
Discussion: https://postgr.es/m/Zmqm9j5EO0I4W8dx@paquier.xyz

doc/src/sgml/xfunc.sgml

@@ -3919,6 +3919,11 @@ extern PgStat_Kind pgstat_add_kind(PgStat_Kind kind,
      type in <xref linkend="guc-shared-preload-libraries"/> so that it will
      be loaded early during <productname>PostgreSQL</productname> startup.
     </para>
+
+    <para>
+     An example describing how to register and use custom statistics can be
+     found in <filename>src/test/modules/injection_points</filename>.
+    </para>
    </sect2>
 
    <sect2 id="extend-cpp">