Add probe for per-allocator statistics

[ollien]

This extends the existing recon.alloc.allocated metric with a breakdown by allocator. We report two groups of metrics

1. by_allocator, which is a break down of the "high level" allocators (binary_alloc, ll_alloc, etc)
2. backing_carriers[_size]/backing_carriers[_size], which is a break down of the "low level" allocators that back the others (both in terms of carriers and size) for mseg_alloc and sys_alloc.