Create Go execution tracer task for each span

This allows us to associate OpenCensus spans with execution
tracer tasks and allow users to have fine-grained details
about the runtime events happened in the lifetime of a
distributed tracing span.

trace/trace.go

@@ -44,6 +44,8 @@ type Span struct {
 	// spanStore is the spanStore this span belongs to, if any, otherwise it is nil.
 	*spanStore
 	exportOnce sync.Once
+
+	executionTracerSpanEnd func() // ends the execution tracer span
 }
 
 // IsRecordingEvents returns true if events are being recorded for this span.
@@ -162,6 +164,9 @@ func StartSpan(ctx context.Context, name string, o ...StartOption) (context.Cont
 		op(&opts)
 	}
 	span := startSpanInternal(name, parent != SpanContext{}, parent, false, opts)
+
+	ctx, end := startExecutionTracerSpan(ctx, name)
+	span.executionTracerSpanEnd = end
 	return NewContext(ctx, span), span
 }
 
@@ -175,6 +180,8 @@ func StartSpanWithRemoteParent(ctx context.Context, name string, parent SpanCont
 		op(&opts)
 	}
 	span := startSpanInternal(name, parent != SpanContext{}, parent, true, opts)
+	ctx, end := startExecutionTracerSpan(ctx, name)
+	span.executionTracerSpanEnd = end
 	return NewContext(ctx, span), span
 }
 
@@ -259,6 +266,7 @@ func (s *Span) End() {
 		return
 	}
 	s.exportOnce.Do(func() {
+		s.executionTracerSpanEnd()
 		// TODO: optimize to avoid this call if sd won't be used.
 		sd := s.makeSpanData()
 		sd.EndTime = internal.MonotonicEndTime(sd.StartTime)

trace/trace_go11.go

@@ -0,0 +1,26 @@
+// Copyright 2018, OpenCensus Authors
+//
+// 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.
+
+// +build go1.11
+
+package trace
+
+import (
+	"context"
+	t "runtime/trace"
+)
+
+func startExecutionTracerSpan(ctx context.Context, name string) (context.Context, func()) {
+	return t.NewContext(ctx, name)
+}

trace/trace_nongo11.go

@@ -0,0 +1,25 @@
+// Copyright 2018, OpenCensus Authors
+//
+// 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.
+
+// +build !go1.11
+
+package trace
+
+import (
+	"context"
+)
+
+func startExecutionTracerSpan(ctx context.Context, name string) (context.Context, func()) {
+	return ctx, func() {}
+}