add CI test to make sure the generated gapic metadata file is parseable

Take baselines/showcase/src/v1beta1/gapic_metadata.json.baseline, make sure it can be parsed by Python protobuf JSON loading code like this one:

```py
from google.protobuf.json_format import Parse
import gapic_metadata_pb2

with open("baselines/showcase/src/v1beta1/gapic_metadata.json.baseline", "r") as f:
metadata = Parse(f.read(), gapic_metadata_pb2.GapicMetadata())
print(metadata)
```