Implement monitored resource for logging client #3377

Log entries can have a monitored resource associated with them as of v2 of the logging API.

• entries.write documentation: https://cloud.google.com/logging/docs/reference/v2/rest/v2/entries/write
• MonitoredResource reference: https://cloud.google.com/logging/docs/reference/v2/rest/v2/MonitoredResource
• Existing tiny resource class in monitoring: https://github.com/GoogleCloudPlatform/google-cloud-python/blob/master/monitoring/google/cloud/monitoring/resource.py#L159

Essentially, the _BaseEntry class should be extend to accept a resource. Helper methods such as log_text and log_struct should likely be extended to accept the resource as well.

This'll need to work with http and gax.