If the DEPOT_TOOLS_REPORT_BUILD envvar is set, Depot Tools will
report information about the builder running the command
(e.g. buildbucket project, bucket, builder and build id).

It will also authenticate to the metrics server, and ignore any
requests not made by ChOps service accounts.

Change-Id: I078a4c2170b4226086c42f289fa449bdebc87179
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/2861213
Commit-Queue: Edward Lesmes <ehmaldonado@chromium.org>
Reviewed-by: Josip Sokcevic <sokcevic@google.com>