compute/metadata: fix example
compute/metadata: fix example

Rewrite the example using the fact that godoc displays all the code in
a single-example file.

Also, fix the copyright date.

Change-Id: I2bc988891c24ea61577165f74c62df18d2b5e43d
Reviewed-on: https://code-review.googlesource.com/29070
Reviewed-by: Jean de Klerk <605b93664484fed1e671a07bff68da39e73d0174@google.com>
Reviewed-by: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>

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