Add stats: arena uptime.

include/jemalloc/internal/arena_structs_b.h

@@ -261,6 +261,8 @@ struct arena_s {
 	 * Synchronization: internal.
 	 */
 	base_t			*base;
+	/* Used to determine uptime.  Read-only after initialization. */
+	nstime_t		create_time;
 };
 
 /* Used in conjunction with tsd for fast arena-related context lookup. */

include/jemalloc/internal/stats.h

@@ -145,6 +145,9 @@ typedef struct arena_stats_s {
 
 	/* One element for each large size class. */
 	malloc_large_stats_t	lstats[NSIZES - NBINS];
+
+	/* Arena uptime. */
+	nstime_t		uptime;
 } arena_stats_t;
 
 #endif /* JEMALLOC_INTERNAL_STATS_H */