GOCBC-264: Added support for OpenTracing tracing.

Motivation
----------
In order to enable more detailed introspection on the behavior
and performance of the SDK, having access to internal trace
metrics is highly valuable.

Changes
-------
Implemented the ability to trace the majority of the internal
methods of the SDK both for KV and non-KV.

Change-Id: Ia3e9b1bc3d62e73ab1688e47efbb104d9bba24a7
Reviewed-on: http://review.couchbase.org/90165
Reviewed-by: Sergey Avseyev <sergey.avseyev@gmail.com>
Tested-by: Brett Lawson <brett19@gmail.com>

auth.go

@@ -1,6 +1,8 @@
 package gocb
 
-import "gopkg.in/couchbase/gocbcore.v7"
+import (
+	"gopkg.in/couchbase/gocbcore.v7"
+)
 
 // UserPassPair represents a username and password pair.
 type UserPassPair gocbcore.UserPassPair

bucket.go

@@ -1,6 +1,7 @@
 package gocb
 
 import (
+	"github.com/opentracing/opentracing-go"
 	"gopkg.in/couchbase/gocbcore.v7"
 	"math/rand"
 	"time"
@@ -13,6 +14,7 @@ type Bucket struct {
 	password  string
 	client    *gocbcore.Agent
 	mtEnabled bool
+	tracer    opentracing.Tracer
 
 	transcoder      Transcoder
 	opTimeout       time.Duration
@@ -26,6 +28,12 @@ type Bucket struct {
 	internal *BucketInternal
 }
 
+func (b *Bucket) startKvOpTrace(operationName string) opentracing.Span {
+	return b.tracer.StartSpan(operationName,
+		opentracing.Tag{Key: "couchbase.bucket", Value: b.name},
+		opentracing.Tag{Key: "couchbase.service", Value: "kv"})
+}
+
 func createBucket(cluster *Cluster, config *gocbcore.AgentConfig) (*Bucket, error) {
 	cli, err := gocbcore.CreateAgent(config)
 	if err != nil {
@@ -39,6 +47,7 @@ func createBucket(cluster *Cluster, config *gocbcore.AgentConfig) (*Bucket, erro
 		client:     cli,
 		mtEnabled:  config.UseMutationTokens,
 		transcoder: &DefaultTranscoder{},
+		tracer:     config.Tracer,
 
 		opTimeout:       2500 * time.Millisecond,
 		bulkOpTimeout:   10000 * time.Millisecond,