fix(firestore): remove excessive spans on iterator (#4163)

Currently the trace span is logged for each iterator.Next()
even if it's not sending an RPC. Therefore, only the first
iteration is actually useful, the rest happens in memory and not
networked.

Fixes #4133.

Signed-off-by: Ahmet Alp Balkan <ahmetb@google.com>

Co-authored-by: Christopher Wilcox <crwilcox@google.com>
Co-authored-by: Chris Cotter <cjcotter@google.com>

firestore/query.go

@@ -701,11 +701,11 @@ func newQueryDocumentIterator(ctx context.Context, q *Query, tid []byte) *queryD
 }
 
 func (it *queryDocumentIterator) next() (_ *DocumentSnapshot, err error) {
-	it.ctx = trace.StartSpan(it.ctx, "cloud.google.com/go/firestore.Query.RunQuery")
-	defer func() { trace.EndSpan(it.ctx, err) }()
-
 	client := it.q.c
 	if it.streamClient == nil {
+		it.ctx = trace.StartSpan(it.ctx, "cloud.google.com/go/firestore.Query.RunQuery")
+		defer func() { trace.EndSpan(it.ctx, err) }()
+
 		sq, err := it.q.toProto()
 		if err != nil {
 			return nil, err