Add Stackdriver Trace exporter for trace. (#160)

* Add Stackdriver Trace exporter for trace.

TODOs for future work is:

* to replace bundler.Bundler

* to add proper tests for the exporter

* to move the exporter to proper repository once it will be created.

* Change to use functions for the exporter initialization instead of
passing option struct directly.
This fix is aliging the same fix for Jaeger (#146, #161)

* Change Option struct to be function type

* Change the original Option struct to be private

* Add line comments to maxMessageEventsPerSpan to leave it for future implementation

* Fix unnessesary expressions specified by `make precommit`

Left errors by `make precommit` in experimental/bridge/opentracing.

* Ran make precommit

* Add new line at EOF

* WIP: Start implementing BatchSpanExporter interfaces

* Change to use RegisterSpanProcessor to register bsp

* Change function names to fit current implementation of sdk

* Removed google.golang.org/api/support/bundler and implement ssp and bsp

* Change spanProcessor as a member of Exporter.

* Fix option names used for BatchSpanProcessor initialization.

* Change Exporter.Shutdown just to unregister spanProcessor.

* Removed copyright statements of OpenCensus.

* Fix small typo and EOF new line

* Fix interfaces of ExportSpan/ExportSpans to meet SpanSyncer/SpanBatcher

* Change to follow context.Context passed in ExportSpan/ExportSpans

* Fix Stackdriver Exporter to hold sync.Once to lock when it is registered and
unregistered.

example/http-stackdriver/server/modd.conf

@@ -0,0 +1,7 @@
+# A basic modd.conf file for Go development.
+
+# Run go test on ALL modules on startup, and subsequently only on modules
+# containing changes.
+server.go {
+    daemon +sigterm: go run server.go
+}

example/http-stackdriver/server/server.go

@@ -0,0 +1,80 @@
+// 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 main
+
+import (
+	"io"
+	"log"
+	"net/http"
+	"os"
+
+	"go.opentelemetry.io/api/tag"
+	"go.opentelemetry.io/api/trace"
+	"go.opentelemetry.io/exporter/trace/stackdriver"
+	"go.opentelemetry.io/plugin/httptrace"
+	sdktrace "go.opentelemetry.io/sdk/trace"
+)
+
+func initTracer() {
+	sdktrace.Register()
+
+	projectID := os.Getenv("PROJECT_ID")
+
+	// Create Stackdriver exporter to be able to retrieve
+	// the collected spans.
+	exporter, err := stackdriver.NewExporter(
+		stackdriver.WithProjectID(projectID),
+	)
+	if err != nil {
+		log.Fatal(err)
+	}
+	if err := exporter.RegisterBatchSpanProcessor(); err != nil {
+		log.Fatal(err)
+	}
+
+	// For the demonstration, use sdktrace.AlwaysSample sampler to sample all traces.
+	// In a production application, use sdktrace.ProbabilitySampler with a desired probability.
+	sdktrace.ApplyConfig(sdktrace.Config{DefaultSampler: sdktrace.AlwaysSample()})
+}
+
+func main() {
+	initTracer()
+
+	helloHandler := func(w http.ResponseWriter, req *http.Request) {
+		attrs, tags, spanCtx := httptrace.Extract(req.Context(), req)
+
+		req = req.WithContext(tag.WithMap(req.Context(), tag.NewMap(tag.MapUpdate{
+			MultiKV: tags,
+		})))
+
+		ctx, span := trace.GlobalTracer().Start(
+			req.Context(),
+			"hello",
+			trace.WithAttributes(attrs...),
+			trace.ChildOf(spanCtx),
+		)
+		defer span.End()
+
+		span.AddEvent(ctx, "handling this...")
+
+		_, _ = io.WriteString(w, "Hello, world!\n")
+	}
+
+	http.HandleFunc("/hello", helloHandler)
+	err := http.ListenAndServe(":7777", nil)
+	if err != nil {
+		panic(err)
+	}
+}