add_cloud_metadata processor overwrites fields when field keys contains dots

[kaiyan-sheng]

In #11612, we realized when overwrite flag equals false in add_cloud_metadata processor, it checks if cloud field exists but can't deal with when field key containing dots for example cloud.provider. Please see TestRetrieveAWSMetadata in provider_aws_ec2_test.go for more details.

This PR is to discuss if add_cloud_metadata processor needs to deal with dots in field keys and if so what's the best implementation here.

[kaiyan-sheng]

In #11612, I tried out some code to check if field key starts with cloud., if so, add_cloud_metadata should not overwrite cloud fields: https://github.com/elastic/beats/blob/38badaa57f14922562520ee38de7b9b5aba0aacf/libbeat/processors/add_cloud_metadata/add_cloud_metadata.go#L372

[jlind23]

Backlog grooming: Closing it for now until further activity, can still be reopened if needed.