options: allow passing in TLS credentials

Allow passing in TLS credentials.
This PR is the sister to ocagent's
census-instrumentation/opencensus-service#415

We shall have the ability to have a secure ocagent and
applications that share the certificate through whatever
certificate sharing mechanism e.g. rotation, file etc.

For an example
```go
func main() {
	// Please take at look at https://godoc.org/google.golang.org/grpc/credentials#TransportCredentials
	// for ways on how to initialize gRPC TransportCredentials.
	creds, err := credentials.NewClientTLSFromFile("my-cert.pem", "")
	if err != nil {
		log.Fatalf("Failed to create gRPC client TLS credentials: %v", err)
	}

	exp, err := ocagent.NewExporter(ocagent.WithTLSCredentials(creds), ocagent.WithServiceName("engine"))
	if err != nil {
		log.Fatalf("Failed to create the agent exporter: %v", err)
	}
	defer exp.Stop()

	// Now register it as a trace exporter.
	trace.RegisterExporter(exp)

	// Then use the OpenCensus tracing library, like we normally would.
	ctx, span := trace.StartSpan(context.Background(), "Securely-Talking-To-Agent-Span")
	defer span.End()

	for i := 0; i < 10; i++ {
		_, iSpan := trace.StartSpan(ctx, fmt.Sprintf("Sample-%d", i))
		<-time.After(6 * time.Millisecond)
		iSpan.End()
	}
}
```

Fixes #44

example_test.go

@@ -20,11 +20,13 @@ import (
 	"log"
 	"time"
 
+	"google.golang.org/grpc/credentials"
+
 	"contrib.go.opencensus.io/exporter/ocagent"
 	"go.opencensus.io/trace"
 )
 
-func Example() {
+func Example_insecure() {
 	exp, err := ocagent.NewExporter(ocagent.WithInsecure(), ocagent.WithServiceName("engine"))
 	if err != nil {
 		log.Fatalf("Failed to create the agent exporter: %v", err)
@@ -44,3 +46,31 @@ func Example() {
 		iSpan.End()
 	}
 }
+
+func Example_withTLS() {
+	// Please take at look at https://godoc.org/google.golang.org/grpc/credentials#TransportCredentials
+	// for ways on how to initialize gRPC TransportCredentials.
+	creds, err := credentials.NewClientTLSFromFile("my-cert.pem", "")
+	if err != nil {
+		log.Fatalf("Failed to create gRPC client TLS credentials: %v", err)
+	}
+
+	exp, err := ocagent.NewExporter(ocagent.WithTLSCredentials(creds), ocagent.WithServiceName("engine"))
+	if err != nil {
+		log.Fatalf("Failed to create the agent exporter: %v", err)
+	}
+	defer exp.Stop()
+
+	// Now register it as a trace exporter.
+	trace.RegisterExporter(exp)
+
+	// Then use the OpenCensus tracing library, like we normally would.
+	ctx, span := trace.StartSpan(context.Background(), "Securely-Talking-To-Agent-Span")
+	defer span.End()
+
+	for i := 0; i < 10; i++ {
+		_, iSpan := trace.StartSpan(ctx, fmt.Sprintf("Sample-%d", i))
+		<-time.After(6 * time.Millisecond)
+		iSpan.End()
+	}
+}

ocagent.go

@@ -23,6 +23,7 @@ import (
 
 	"google.golang.org/api/support/bundler"
 	"google.golang.org/grpc"
+	"google.golang.org/grpc/credentials"
 	"google.golang.org/grpc/metadata"
 
 	"go.opencensus.io/resource"
@@ -82,6 +83,8 @@ type Exporter struct {
 	// from OpenCensus-Go view.Data to metricspb.Metric.
 	// Please do not confuse it with metricsBundler!
 	viewDataBundler *bundler.Bundler
+
+	clientTransportCredentials credentials.TransportCredentials
 }
 
 func NewExporter(opts ...ExporterOption) (*Exporter, error) {
@@ -257,7 +260,9 @@ func (ae *Exporter) createMetricsServiceConnection(cc *grpc.ClientConn, node *co
 func (ae *Exporter) dialToAgent() (*grpc.ClientConn, error) {
 	addr := ae.prepareAgentAddress()
 	var dialOpts []grpc.DialOption
-	if ae.canDialInsecure {
+	if ae.clientTransportCredentials != nil {
+		dialOpts = append(dialOpts, grpc.WithTransportCredentials(ae.clientTransportCredentials))
+	} else if ae.canDialInsecure {
 		dialOpts = append(dialOpts, grpc.WithInsecure())
 	}
 	if ae.compressor != "" {

options.go

@@ -14,7 +14,11 @@
 
 package ocagent
 
-import "time"
+import (
+	"time"
+
+	"google.golang.org/grpc/credentials"
+)
 
 const (
 	DefaultAgentPort uint16 = 55678
@@ -103,3 +107,22 @@ func (h headerSetter) withExporter(e *Exporter) {
 func WithHeaders(headers map[string]string) ExporterOption {
 	return headerSetter(headers)
 }
+
+type clientCredentials struct {
+	credentials.TransportCredentials
+}
+
+var _ ExporterOption = (*clientCredentials)(nil)
+
+// WithTLSCredentials allows the connection to use TLS credentials
+// when talking to the server. It takes in grpc.TransportCredentials instead
+// of say a Certificate file or a tls.Certificate, because the retrieving
+// these credentials can be done in many ways e.g. plain file, in code tls.Config
+// or by certificate rotation, so it is up to the caller to decide what to use.
+func WithTLSCredentials(creds credentials.TransportCredentials) ExporterOption {
+	return &clientCredentials{TransportCredentials: creds}
+}
+
+func (cc *clientCredentials) withExporter(e *Exporter) {
+	e.clientTransportCredentials = cc.TransportCredentials
+}