Collect the hostname of the machine as a metric.

The hostname is collected during a build to determine where the user
is build from (from office workstation, cloudtop, etc...). This is a
useful metrics for RBE related builds.

Bug: b/169395325
Test: m nothing, ran printproto to verify that soong_metrics has the
      hostname.
Change-Id: I3e5d64f948ae43c5dda60de1d4a52b187e36dff8