spanner: adjust where trace span is created for Apply
spanner: adjust where trace span is created for Apply

It should cover both regular and at-least-once applies, or else there
is no span that covers the former.

Fixes #1750

Change-Id: I2d60e74fe0c4ce6c5283fefcc551eda8f6ff21a3
Reviewed-on: https://code-review.googlesource.com/c/gocloud/+/50910
Reviewed-by: Knut Olav Løite <62634d14bf6dff6edb4b7af974862bb227095f71@gmail.com>
Reviewed-by: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>

googleapis/google-cloud-go,googleapis/google-cloud-go,googleapis/google-cloud-go,googleapis/google-cloud-go