Metrics label generation, including bookshelf

These functions used to be in chef_wm's chef_wm_base (and
corresponding modules for Private Chef).  They are extracted here in a
common location to reduce duplication between Chef server varieties.

Additionally, support for generating bookshelf / s3 related metric
labels is added.

Tests are included, and Dialyzer passes.
Merge branch 'cm/OC-870/s3_instrumentation'