bundling does not work properly on logging

[jmuk]

I'm doing verification of logging API, and noticed a buggy behavior; when the logging client does not specify all of the descriminator_fields https://github.com/googleapis/googleapis/blob/master/google/logging/v2/logging_gapic.yaml#L356, it fails to bundle the request, and it rejects to create API calls. Those fields are said optional in the proto file (https://github.com/googleapis/googleapis/blob/master/google/logging/v2/logging.proto#L85).

There are multiple bugs here:

• when discriminator fields are partially specified, it should bundle the requests upon the request parameters as much as possible.
• when such fields are not specified at all, it should not cause any errors. It should fall back to a normal API call instead.