Add stats for metadata_thp.

Report number of THPs used in arena and aggregated stats.

include/jemalloc/internal/ctl.h

@@ -48,6 +48,7 @@ typedef struct ctl_stats_s {
 	size_t allocated;
 	size_t active;
 	size_t metadata;
+	size_t metadata_thp;
 	size_t resident;
 	size_t mapped;
 	size_t retained;