PG-1907 Use macro to define the number of enum values

This commet address refactoring made in PostgreSQL codebase in commit
10b7218.

pg_stat_monitor.h

@@ -152,10 +152,10 @@ typedef enum pgsmStoreKind
 	PGSM_EXEC,
 	PGSM_STORE,
 	PGSM_ERROR,
-
-	PGSM_NUMKIND				/* Must be last value of this enum */
 } pgsmStoreKind;
 
+#define PGSM_NUMKIND	(PGSM_ERROR + 1)
+
 /* the assumption of query max nested level */
 #define DEFAULT_MAX_NESTED_LEVEL	10