interceptor: make endToEnd test an example test (#91)

Previously:
    TestNewInterceptor_endToEnd

served the purpose of demoing how to make an "agent"
with the OpenCensus interceptor. It just spams stdout
with output and doesn't perform any assertions
nor serve any other purpose other than demoing.
As we add other interceptors, we'll put them in
here. Instead make it an example test that compiles
and will serve as the blueprint towards making
cmd/ocagent modifications.

Author: Emmanuel T Odeke

interceptor/end_to_end_test.go

@@ -18,7 +18,6 @@ import (
 	"context"
 	"encoding/json"
 	"log"
-	"testing"
 	"time"
 
 	"contrib.go.opencensus.io/exporter/ocagent"
@@ -31,7 +30,7 @@ import (
 	"github.com/census-instrumentation/opencensus-service/spanreceiver"
 )
 
-func TestNewInterceptor_endToEnd(t *testing.T) {
+func Example_endToEnd() {
 	// This is what the cmd/ocagent code would look like this.
 	// A trace interceptor as per the trace interceptor
 	// configs that have been parsed.
@@ -58,7 +57,7 @@ func TestNewInterceptor_endToEnd(t *testing.T) {
 			_ = ti.StopTraceInterception(context.Background())
 		}
 	}()
-	t.Log("Done starting the trace interceptor")
+	log.Println("Done starting the trace interceptor")
 	// We are done with the agent-core
 
 	// Now this code would exist in the client application e.g. client code.
@@ -74,7 +73,7 @@ func TestNewInterceptor_endToEnd(t *testing.T) {
 	// For demo purposes we are always sampling
 	trace.ApplyConfig(trace.Config{DefaultSampler: trace.AlwaysSample()})
 
-	t.Log("Starting loop")
+	log.Println("Starting loop")
 	ctx, span := trace.StartSpan(context.Background(), "ClientLibrarySpan")
 	for i := 0; i < 10; i++ {
 		_, span := trace.StartSpan(ctx, "ChildSpan")