stats_hero instrumentation of Bookshelf / S3 upstream

This allows Erchef to log metrics on Bookshelf / S3 operations
(currently `chef_s3:check_checksums/2` and
`chef_s3:delete_checksums/2`).

Metric labels differ from existing labels in that they incorporate
both the storage hostname and storage bucket, in addition to the
standard upstream, module, and function names.

To facilitate this, label generation was extracted into a new module,
`chef_metrics` in the `chef_objects` application.