[META 826] changes to `metadata.cloud.project.*` fields collected for GCP

[elastic-apm-tech]

See meta issue for the description and details:

• Meta issue: https://github.com/elastic/apm/issues/826

[trentm]

It looks like the PHP agent doesn't yet collect cloud metadata. There is https://github.com/elastic/apm-agent-php/issues/76 for that, so this issue isn't necessary.