logging: update "Grouping Logs by Request" docs
logging: update "Grouping Logs by Request" docs

Fixes: 1598
Change-Id: Ibdb39720230286221d1971d948d7d9764dddaa6f
Reviewed-on: https://code-review.googlesource.com/c/gocloud/+/46850
Reviewed-by: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>
Reviewed-by: Jean de Klerk <605b93664484fed1e671a07bff68da39e73d0174@google.com>

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