[WIP] Reduce number of allocations in histogram metric

[simpl1g]

@dmagliola Hi. I'm committing this as WIP so you can compare old and new implementations side by side, maybe you can add some test cases for benchmarks to find problems. If you are ok with changes I will clean up everything. All tests are passing without modifications, I guess nothing will change in logic it self. I haven't touched #get method though, where is it used?

You can find bench results at the end of spec/benchmarks/histogram.rb file. There are some improvements, but I'm not really happy with results. I believe storage should be changed a bit. Current way of storing each :le as separate hash requires a lot of manipulations for #values. Having single hash can be more beneficial, but current store interface does not allow do this.

Instead of

{{l1: '1', le: '1'} => 1, {l1: '1', le: '2'} => 1, {l1: '1', le: 'sum'} => 2}

something like this should be easier to operate

{{l1: '1'} =>  {'1' => 1, '2' => 1, 'sum' => 2}}

[simpl1g]

I will also remove viernier profiler, it is for newer versions of ruby