Include TARGET_RELEASE in soong metrics

While we explicitly do not tell the build engine the value of
TARGET_RELEASE, we can include it in the build metrics.

Bug: None
Test: Manual
Change-Id: I92e4b21b8c93e1039d405d29eaebfa3b89a57103
diff --git a/ui/build/dumpvars.go b/ui/build/dumpvars.go
index e0b2c08..b592f11 100644
--- a/ui/build/dumpvars.go
+++ b/ui/build/dumpvars.go
@@ -137,6 +137,12 @@
 		}
 	}
 	if ctx.Metrics != nil {
+		// Also include TARGET_RELEASE in the metrics.  Do this first
+		// so that it gets overwritten if dumpvars ever spits it out.
+		if release, found := os.LookupEnv("TARGET_RELEASE"); found {
+			ctx.Metrics.SetMetadataMetrics(
+				map[string]string{"TARGET_RELEASE": release})
+		}
 		ctx.Metrics.SetMetadataMetrics(ret)
 	}