Automatically generate .repo-metadata.json

[parthea]

In PR googleapis/google-cloud-python#11808, the api_id field in packages/google-shopping-merchant-inventories/.repo-metadata.json is set to inventories.googleapis.com. This is incorrect. The correct api_id should be merchantapi.googleapis.com. See https://github.com/googleapis/googleapis/blob/2c12a275b10a2f04926a1505a5fd1b8acf4a6833/google/shopping/merchant/inventories/v1beta/merchantapi_v1beta.yaml#L3C7-L3C33

[parthea]

I'm going to transfer this issue to gapic-generator-python as the problem would be solved if the gapic-generator could autogenerate .repo-metadata.json using information in the service YAML file.
https://github.com/googleapis/googleapis/blob/master/google/shopping/merchant/inventories/v1beta/merchantapi_v1beta.yaml#L3