Rename time to event in Soong metrics system.

Several structs and files are named time which records the total
execution of a traceable event. Soon, resource information of an
executed process is collected during a build event so a more generic
name is applied.

Bug: b/169453825
Test: go test and m nothing. Checked the soong_metrics protobuf.
Change-Id: I65a782603530d51018d6cc1192c18cfeef7566a0

ui/metrics/event.go

@@ -0,0 +1,69 @@
+// Copyright 2018 Google Inc. All Rights Reserved.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//   http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package metrics
+
+import (
+	"time"
+
+	"android/soong/ui/metrics/metrics_proto"
+	"android/soong/ui/tracer"
+	"github.com/golang/protobuf/proto"
+)
+
+// for testing purpose only
+var _now = now
+
+type event struct {
+	desc string
+	name string
+
+	// the time that the event started to occur.
+	start time.Time
+}
+
+type EventTracer interface {
+	Begin(name, desc string, thread tracer.Thread)
+	End(thread tracer.Thread) soong_metrics_proto.PerfInfo
+}
+
+type eventTracerImpl struct {
+	activeEvents []event
+}
+
+var _ EventTracer = &eventTracerImpl{}
+
+func now() time.Time {
+	return time.Now()
+}
+
+func (t *eventTracerImpl) Begin(name, desc string, _ tracer.Thread) {
+	t.activeEvents = append(t.activeEvents, event{name: name, desc: desc, start: _now()})
+}
+
+func (t *eventTracerImpl) End(tracer.Thread) soong_metrics_proto.PerfInfo {
+	if len(t.activeEvents) < 1 {
+		panic("Internal error: No pending events for endAt to end!")
+	}
+	lastEvent := t.activeEvents[len(t.activeEvents)-1]
+	t.activeEvents = t.activeEvents[:len(t.activeEvents)-1]
+	realTime := uint64(_now().Sub(lastEvent.start).Nanoseconds())
+
+	return soong_metrics_proto.PerfInfo{
+		Desc:      proto.String(lastEvent.desc),
+		Name:      proto.String(lastEvent.name),
+		StartTime: proto.Uint64(uint64(lastEvent.start.UnixNano())),
+		RealTime:  proto.Uint64(realTime),
+	}
+}