Description

https://github.com/confluentinc/confluent-kafka-python/blob/master/confluent_kafka/schema_registry/protobuf.py#L315
_known_subject is never populated.

The ProtobufSerializer has a cache for the subject created. However, it is never populated in the code. As such, the serializer does a round trip to the schema registry upon every serialization to lookup the schema by calling the lookup function here
https://github.com/confluentinc/confluent-kafka-python/blob/master/confluent_kafka/schema_registry/protobuf.py#L323

How to reproduce

Try to serialize multiple times with the same serializer instance and look at the schema registry logs.

Checklist

Please provide the following information:

• confluent-kafka-python and librdkafka version (confluent_kafka.version() and confluent_kafka.libversion()):
• Apache Kafka broker version:
• Client configuration: {...}
• Operating system:
• Provide client logs (with 'debug': '..' as necessary)
• Provide broker log excerpts
• Critical issue