fix: simplify jemalloc metrics (#23)

Author: heilhead

crates/alloc/src/stats.rs

@@ -137,6 +137,11 @@ pub fn update_jemalloc_metrics() -> Result<(), Error> {
         // Cumulative number of allocation requests satisfied by bin regions of the
         // corresponding size class.
         gauge("jemalloc_memory_bin_nrequests", bin_stats.nrequests);
+
+        let active = bin_stats.nmalloc - bin_stats.ndalloc;
+
+        gauge("jemalloc_memory_bin_nactive", active);
+        gauge("jemalloc_memory_bin_nactive_size", active * bin_const.size);
     }
 
     Ok(())