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

ui/metrics/metrics_proto/metrics.proto

@@ -89,6 +89,9 @@ message MetricsBase {
 
   // The metrics for calling Ninja.
   repeated PerfInfo ninja_runs = 20;
+
+  // The metrics for the whole build
+  optional PerfInfo total = 21;
 }
 
 message PerfInfo {