Testing for K8s Metadata Watcher

[jlewi]

We need a good way to test the K8s metadata watchers implementation with MLMD.

Two ways that come to mind are

1. Creating a fake implementation of the MLMD gRPC meta data [store service](https://github.com/google/ml-metadata/blob/1772ee39766c3c02dba4b3e3b43a8745b2eb58af/ml_metadata/proto/metadata_store_service.proto#L550
2. Running MLMD in a subprocess possibly using sqllite
3. Running MLMD in a docker container

It looks like MLMD is developing go client libraries and we can take a look at how that testing is being done. https://github.com/google/ml-metadata/blob/master/ml_metadata/metadata_store/metadata_store_test.go