Add and update comments in ui/metrics/event.go

The event.go code was refactored to follow the best practices
of golang code style.

Bug: b/172918681
Test: go test ./ui/metrics
Change-Id: Ic57d7cd576a9b146530ae37f03f4f5679d6cffef
diff --git a/ui/metrics/metrics.go b/ui/metrics/metrics.go
index 7031042..2a09461 100644
--- a/ui/metrics/metrics.go
+++ b/ui/metrics/metrics.go
@@ -39,13 +39,13 @@
 
 type Metrics struct {
 	metrics     soong_metrics_proto.MetricsBase
-	EventTracer EventTracer
+	EventTracer *EventTracer
 }
 
 func New() (metrics *Metrics) {
 	m := &Metrics{
 		metrics:     soong_metrics_proto.MetricsBase{},
-		EventTracer: &eventTracerImpl{},
+		EventTracer: &EventTracer{},
 	}
 	return m
 }