feat(storage): add trace span to Writer (#8375)

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

Fixes #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

@@ -22,6 +22,8 @@ import (
 	"sync"
 	"time"
 	"unicode/utf8"
+
+	"cloud.google.com/go/internal/trace"
 )
 
 // A Writer writes a Cloud Storage object.
@@ -163,6 +165,7 @@ func (w *Writer) Close() error {
 	<-w.donec
 	w.mu.Lock()
 	defer w.mu.Unlock()
+	trace.EndSpan(w.ctx, w.err)
 	return w.err
 }