preview/logging: add a timestamp filter example
preview/logging: add a timestamp filter example

Add an example showing how to filter by timestamp using a Go time.Time.

Fixes #344.

Change-Id: I939dac03fabe2f8104fd06cd135931f2b73a4c86
Reviewed-on: https://code-review.googlesource.com/7441
Reviewed-by: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>

michael-grosshans/google-cloud-go,googleapis/google-cloud-go,googleapis/google-cloud-go,michael-grosshans/google-cloud-go,googleapis/google-cloud-go,googleapis/google-cloud-go