Move opencensus example to the example directory (#1359)

And update .gitignore, so the built binary does not show up in git
status.

example/opencensus/main.go

@@ -0,0 +1,58 @@
+// Copyright The 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 "go.opentelemetry.io/otel/bridge/opencensus/examples/simple"
+
+import (
+	"context"
+	"log"
+
+	octrace "go.opencensus.io/trace"
+
+	"go.opentelemetry.io/otel"
+	"go.opentelemetry.io/otel/bridge/opencensus"
+	"go.opentelemetry.io/otel/exporters/stdout"
+	sdktrace "go.opentelemetry.io/otel/sdk/trace"
+)
+
+func main() {
+	ctx := context.Background()
+
+	log.Println("Configuring OpenCensus.  Not Registering any OpenCensus exporters.")
+	octrace.ApplyConfig(octrace.Config{DefaultSampler: octrace.AlwaysSample()})
+
+	log.Println("Registering OpenTelemetry stdout exporter.")
+	otExporter, err := stdout.NewExporter(stdout.WithPrettyPrint())
+	if err != nil {
+		log.Fatal(err)
+	}
+	tp := sdktrace.NewTracerProvider(sdktrace.WithSyncer(otExporter))
+	otel.SetTracerProvider(tp)
+
+	log.Println("Installing the OpenCensus bridge to make OpenCensus libraries write spans using OpenTelemetry.")
+	tracer := tp.Tracer("simple")
+	octrace.DefaultTracer = opencensus.NewTracer(tracer)
+
+	log.Println("Creating OpenCensus span, which should be printed out using the OpenTelemetry stdout exporter.\n-- It should have no parent, since it is the first span.")
+	ctx, outerOCSpan := octrace.StartSpan(ctx, "OpenCensusOuterSpan")
+	outerOCSpan.End()
+
+	log.Println("Creating OpenTelemetry span\n-- It should have the OpenCensus span as a parent, since the OpenCensus span was written with using OpenTelemetry APIs.")
+	ctx, otspan := tracer.Start(ctx, "OpenTelemetrySpan")
+	otspan.End()
+
+	log.Println("Creating OpenCensus span, which should be printed out using the OpenTelemetry stdout exporter.\n-- It should have the OpenTelemetry span as a parent, since it was written using OpenTelemetry APIs")
+	_, innerOCSpan := octrace.StartSpan(ctx, "OpenCensusInnerSpan")
+	innerOCSpan.End()
+}