Include OpenCensus base library version in user-agent header (#11)

Fixes: #7

Author: Ramon Nogueira

Gopkg.lock

@@ -27,15 +27,15 @@
 
 [[constraint]]
   name = "cloud.google.com/go"
-  version = ">=0.23.0, <1.0.0"
+  version = ">=0.23.0"
 
 [[constraint]]
   name = "github.com/golang/protobuf"
   version = "1.1.0"
 
 [[constraint]]
   name = "go.opencensus.io"
-  version = ">=0.9.0, <1.0.0"
+  version = ">=0.12.0"
 
 [[constraint]]
   branch = "master"

Gopkg.toml

@@ -32,6 +32,7 @@ import (
 
 	"cloud.google.com/go/monitoring/apiv3"
 	"github.com/golang/protobuf/ptypes/timestamp"
+	"go.opencensus.io/exporterutil"
 	"google.golang.org/api/option"
 	"google.golang.org/api/support/bundler"
 	distributionpb "google.golang.org/genproto/googleapis/api/distribution"
@@ -47,10 +48,11 @@ const (
 	opencensusTaskKey         = "opencensus_task"
 	opencensusTaskDescription = "Opencensus task identifier"
 	defaultDisplayNamePrefix  = "OpenCensus"
-
-	userAgent = "opencensus-go [0.8.0]"
+	version                   = "0.3.0"
 )
 
+var userAgent = fmt.Sprintf("opencensus-go %s; stackdriver-exporter %s", exporterutil.Version, version)
+
 // statsExporter exports stats to the Stackdriver Monitoring.
 type statsExporter struct {
 	bundler *bundler.Bundler