Measure the transfer size and duration of context transfers for various
categories of local source transfers from the progress stream that's
returned during the build.
Local source transfers are split into one of four categories:
* context
* dockerfile
* dockerignore
* namedcontext
Named contexts that are different names will be categorized under the
same metric.
Signed-off-by: Jonathan A. Sternberg <jonathan.sternberg@docker.com>
