"item_alloc_sizes" not correct when using append

Setup a test to constantly append to an item until it reached 20mb, yet the output of timings for item_alloc_sizes only showed 32-64k. I presume this is because it only tracks the incoming value size, but that doesn't work for append/prepend/incr/decr.

Could this be extended to actually get the size of the value for those specific operations or will that be too invasive?