Lower bounds related to pgstats kinds

This commit changes stats kinds to have the following bounds, making
their handling in core cheaper by default:
- PGSTAT_KIND_CUSTOM_MIN 128 -> 24
- PGSTAT_KIND_MAX 256 -> 32

The original numbers were rather high, and showed an impact on
performance in pgstat_report_stat() for the case of simple queries with
its early-exit path if there are no pending statistics to flush.  This
logic will be improved more in a follow-up commit to bring the
performance of pgstat_report_stat() on par with v17 and older versions.
Lowering the bounds is a change worth doing on its own, independently of
the other improvement.

These new numbers should be enough to leave some room for the following
years for built-in and custom stats kinds, with stable ID numbers.  At
least that should be enough to start with this facility for extension
developers.  It can be always increased in the tree depending on the
requirements wanted.

Per discussion with Andres Freund and Bertrand Drouvot.

Discussion: https://postgr.es/m/eb224uegsga2hgq7dfq3ps5cduhpqej7ir2hjxzzozjthrekx5@dysei6buqthe
Backpatch-through: 18

src/include/utils/pgstat_kind.h

@@ -18,7 +18,7 @@
 
 /* Range of IDs allowed, for built-in and custom kinds */
 #define PGSTAT_KIND_MIN	1		/* Minimum ID allowed */
-#define PGSTAT_KIND_MAX	256		/* Maximum ID allowed */
+#define PGSTAT_KIND_MAX	32		/* Maximum ID allowed */
 
 /* use 0 for INVALID, to catch zero-initialized data */
 #define PGSTAT_KIND_INVALID 0
@@ -46,7 +46,7 @@
 /* Custom stats kinds */
 
 /* Range of IDs allowed for custom stats kinds */
-#define PGSTAT_KIND_CUSTOM_MIN	128
+#define PGSTAT_KIND_CUSTOM_MIN	24
 #define PGSTAT_KIND_CUSTOM_MAX	PGSTAT_KIND_MAX
 #define PGSTAT_KIND_CUSTOM_SIZE	(PGSTAT_KIND_CUSTOM_MAX - PGSTAT_KIND_CUSTOM_MIN + 1)
 
@@ -55,7 +55,7 @@
  * development and have not reserved their own unique kind ID yet. See:
  * https://wiki.postgresql.org/wiki/CustomCumulativeStats
  */
-#define PGSTAT_KIND_EXPERIMENTAL	128
+#define PGSTAT_KIND_EXPERIMENTAL	24
 
 static inline bool
 pgstat_is_kind_builtin(PgStat_Kind kind)

src/test/modules/injection_points/injection_stats.c

@@ -59,7 +59,7 @@ static const PgStat_KindInfo injection_stats = {
 /*
  * Kind ID reserved for statistics of injection points.
  */
-#define PGSTAT_KIND_INJECTION	129
+#define PGSTAT_KIND_INJECTION	25
 
 /* Track if stats are loaded */
 static bool inj_stats_loaded = false;

src/test/modules/injection_points/injection_stats_fixed.c

@@ -64,7 +64,7 @@ static const PgStat_KindInfo injection_stats_fixed = {
 /*
  * Kind ID reserved for statistics of injection points.
  */
-#define PGSTAT_KIND_INJECTION_FIXED	130
+#define PGSTAT_KIND_INJECTION_FIXED	26
 
 /* Track if fixed-numbered stats are loaded */
 static bool inj_fixed_loaded = false;