[WIP] Initial HNSW algorithm metrics using tracing library

[bgillesp]

WIP PR implements basic Visitors to count the numbers of distance evaluations, distance comparisons, node opening, and layer search operations which take place during a query. Additionally adds a binary which tallies these counts over the creation of an index of a particular size.

Steps before this PR is ready:

• Polish the existing implementation and add support for any additional basic algorithm metrics that could be useful
• Expose collected metrics to the Python interface to aid in data analysis tasks