Log whether the build is using soong-only mode to the metrics.

Bug: 391952685
Test: gqui -q from out/soong_metrics proto build/soong/ui/metrics/metrics_proto/metrics.proto:MetricsBase | grep soong_only
Change-Id: Iff8ecb34813a88dad8fab93304093ae3ec9bc8c2
diff --git a/ui/build/config.go b/ui/build/config.go
index 890ed46..fdd15ff 100644
--- a/ui/build/config.go
+++ b/ui/build/config.go
@@ -639,6 +639,7 @@
 		UseRbe:                proto.Bool(config.UseRBE()),
 		NinjaWeightListSource: getNinjaWeightListSourceInMetric(config.NinjaWeightListSource()),
 		SoongEnvVars:          soongEnvVars,
+		SoongOnly:             proto.Bool(config.soongOnlyRequested),
 	}
 	c.Targets = append(c.Targets, config.arguments...)