Use mapping metadata for unit rather than field name

Look at removing well known units from the metric names, and instead set them as field metadata for [automatically selecting the default formatter in Lens](https://github.com/elastic/kibana/issues/82348).

_Originally posted by @axw in https://github.com/elastic/gobench/pull/1#pullrequestreview-938783410_

The issue names no files or tests. Start by locating where metric names and mapping metadata are assembled in the Go repository, then review the linked Kibana issue for the expected Lens formatter behavior; done means units are represented in field metadata rather than metric names.

Written by the indexing model from the issue text.