SDK: span processor interface and simple span processor. (#117)

* SDK: SpanProcessor Interface.

* add simple span processor.

* rename span processor.

* fix logic to export or process span data.

sdk/trace/simple_span_processor.go

@@ -0,0 +1,47 @@
+// Copyright 2019, OpenTelemetry 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.
+
+package trace
+
+// SimpleSpanProcessor implements SpanProcessor interfaces. It is used by
+// exporters to receive SpanData synchronously when span is finished.
+type SimpleSpanProcessor struct {
+	exporter Exporter
+}
+
+var _ SpanProcessor = (*SimpleSpanProcessor)(nil)
+
+// NewSimpleSpanProcessor creates a new instance of SimpleSpanProcessor
+// for a given exporter.
+func NewSimpleSpanProcessor(exporter Exporter) *SimpleSpanProcessor {
+	ssp := &SimpleSpanProcessor{
+		exporter: exporter,
+	}
+	return ssp
+}
+
+// OnStart method does nothing.
+func (ssp *SimpleSpanProcessor) OnStart(sd *SpanData) {
+}
+
+// OnEnd method exports SpanData using associated exporter.
+func (ssp *SimpleSpanProcessor) OnEnd(sd *SpanData) {
+	if ssp.exporter != nil {
+		ssp.exporter.ExportSpan(sd)
+	}
+}
+
+// Shutdown method does nothing. There is no data to cleanup.
+func (ssp *SimpleSpanProcessor) Shutdown() {
+}