For PProf.Allocs, change the default metric to `allocs`, not `size`.

[NHDaly]

We default to allocs, since julia's Profile.Allocs code currently uniformly samples accross allocations, so allocs is a representative profile, while size is not.

Explanation:

If each allocation was uniform (as in the case of the allocs sample type), then uniformly sampling should give an accurate distribution of where the allocations occur: code that allocates more often is more likely to show up in the profile.

But if the metric we're interested in is not uniform: how many bytes are allocated, uniform sampling will not get us a representative distribution of where the bytes are allocated! Code that allocates very large objects is no more or less likely to be included than code that allocates very small ones.

[codecov-commenter]

Codecov Report

Merging #55 (003440b) into master (18d842d) will not change coverage. The diff coverage is n/a.

@@           Coverage Diff           @@
##           master      #55   +/-   ##
=======================================
  Coverage   72.81%   72.81%           
=======================================
  Files           3        3           
  Lines         298      298           
=======================================
  Hits          217      217           
  Misses         81       81           

Impacted Files	Coverage Δ
src/Allocs.jl	0.00% <ø> (ø)

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