feat(storage): add trace span to Writer

Adds a trace span that covers a Writer from open to close.
Nested spans already cover individual RPCs that the Writer
makes.

Fixes googleapis#6144

storage/storage.go

@@ -1036,6 +1036,7 @@ func (o *ObjectHandle) ReadCompressed(compressed bool) *ObjectHandle {
 // It is the caller's responsibility to call Close when writing is done. To
 // stop writing without saving the data, cancel the context.
 func (o *ObjectHandle) NewWriter(ctx context.Context) *Writer {
+	ctx = trace.StartSpan(ctx, "cloud.google.com/go/storage.Object.Writer")
 	return &Writer{
 		ctx:         ctx,
 		o:           o,

storage/writer.go

@@ -15,6 +15,7 @@
 package storage
 
 import (
+	"cloud.google.com/go/internal/trace"
 	"context"
 	"errors"
 	"fmt"
@@ -163,6 +164,7 @@ func (w *Writer) Close() error {
 	<-w.donec
 	w.mu.Lock()
 	defer w.mu.Unlock()
+	trace.EndSpan(w.ctx, w.err)
 	return w.err
 }