injection_points: Tweak variable-numbered stats to work with pending data

As coded, the module was not using pending entries to store its data
locally before doing a flush to the central dshash with a timed
pgstat_report_stat() call.  Hence, the flush callback was defined, but
finished by being not used.  As a template, this is more efficient than
the original logic of updating directly the shared memory entries as
this reduces the interactions that need to be done with the pgstats
hash table in shared memory.

injection_stats_flush_cb() was also missing a pgstat_unlock_entry(), so
add one, while on it.

Reviewed-by: Bertrand Drouvot
Discussion: https://postgr.es/m/Z3JbLhKFFm6kKfT8@ip-10-97-1-34.eu-west-3.compute.internal

src/test/modules/injection_points/injection_stats.c

@@ -80,6 +80,9 @@ injection_stats_flush_cb(PgStat_EntryRef *entry_ref, bool nowait)
 		return false;
 
 	shfuncent->stats.numcalls += localent->numcalls;
+
+	pgstat_unlock_entry(entry_ref);
+
 	return true;
 }
 
@@ -127,13 +130,13 @@ pgstat_create_inj(const char *name)
 	if (!inj_stats_loaded || !inj_stats_enabled)
 		return;
 
-	entry_ref = pgstat_get_entry_ref_locked(PGSTAT_KIND_INJECTION, InvalidOid,
-											PGSTAT_INJ_IDX(name), false);
+	entry_ref = pgstat_prep_pending_entry(PGSTAT_KIND_INJECTION, InvalidOid,
+										  PGSTAT_INJ_IDX(name), NULL);
+
 	shstatent = (PgStatShared_InjectionPoint *) entry_ref->shared_stats;
 
 	/* initialize shared memory data */
 	memset(&shstatent->stats, 0, sizeof(shstatent->stats));
-	pgstat_unlock_entry(entry_ref);
 }
 
 /*
@@ -168,16 +171,14 @@ pgstat_report_inj(const char *name)
 	if (!inj_stats_loaded || !inj_stats_enabled)
 		return;
 
-	entry_ref = pgstat_get_entry_ref_locked(PGSTAT_KIND_INJECTION, InvalidOid,
-											PGSTAT_INJ_IDX(name), false);
+	entry_ref = pgstat_prep_pending_entry(PGSTAT_KIND_INJECTION, InvalidOid,
+										  PGSTAT_INJ_IDX(name), NULL);
 
 	shstatent = (PgStatShared_InjectionPoint *) entry_ref->shared_stats;
 	statent = &shstatent->stats;
 
 	/* Update the injection point statistics */
 	statent->numcalls++;
-
-	pgstat_unlock_entry(entry_ref);
 }
 
 /*