Add stats for the size of extent_avail heap

include/jemalloc/internal/arena_stats.h

@@ -87,6 +87,9 @@ struct arena_stats_s {
 	 */
 	atomic_zu_t		retained; /* Derived. */
 
+	/* Number of extent_t structs allocated by base, but not being used. */
+	atomic_zu_t		extent_avail;
+
 	arena_stats_decay_t	decay_dirty;
 	arena_stats_decay_t	decay_muzzy;
 

include/jemalloc/internal/arena_structs_b.h

@@ -196,6 +196,7 @@ struct arena_s {
 	 * Synchronization: extent_avail_mtx.
 	 */
 	extent_tree_t		extent_avail;
+	atomic_zu_t		extent_avail_cnt;
 	malloc_mutex_t		extent_avail_mtx;
 
 	/*