datastore: add attribute to trace

[nktks]

Is your feature request related to a problem? Please describe. When use stackdriver exporter and call datastore client public func (like datastore *Client.Get ), child span ( like cloud.google.com/go/datastore.*) added to our parent context(span). But, added child span has no information like called datastore.Key, datastore.Query, etc... It's more convenient if we can know which key or query related to child span.

Describe the solution you'd like How about calling *Span.AddAttributes when child span started in datastore package?

Describe alternatives you've considered

• Is there any reason (like security reason) that google-cloud-go can't add attribute to stackdriver trace automaticaly?
  • If answer is yes, how about adding explicitly interface to add attribute?

Additional context

• Similarly, I have same feature request for other storage (like spanner)
• I tried to execute local test that I modified datastore.go to use trace.TracePrintf ( internal used trace.Annotatef ), but nothing changed at stackdriver trace label's output.

 func (c *Client) Get(ctx context.Context, key *Key, dst interface{}) (err error) {
        ctx = trace.StartSpan(ctx, "cloud.google.com/go/datastore.Get")
        defer func() { trace.EndSpan(ctx, err) }()
+       trace.TracePrintf(ctx, map[string]interface{}{"Key": key.String()}, "my test")

• I found attribute size limits , so it's difficult to add long attribute text like query string .