Add total build time to metrics

Determining the total build time from the existing metrics is
tricky because some of the metrics are overlapping.  For example,
the "soong" metric includes  "blueprint bootstrap",
"environment check", "minibp", "bpglob", "minibootstrap", and
"bootstrap".  Create a new singleton "total" metric that
covers the interesting parts of the build.

Test: m nothing
Change-Id: Iedcf5c9cf0d27252b694d35e6ff66ca1fcf734ac