KafkaOutput binding 'topic' attribute doesn't support environment variables

[meken]

At least for the Java bindings the following config fails:

@KafkaOutput(topic="%KAFKA_TOPIC%", ...

whereas the username, password etc support environment variables. Interestingly, the @KafkaTrigger topic attribute also supports environment variables.

See below the error messsage indicating that the topic name is not resolved properly.

[2020-08-10 13:58:00] Failed to delivery message to %KAFKA_TOPIC% / -1 / -1001. Reason: Local: Unknown topic. Full Error: Local: Unknown topic
[2020-08-10 13:58:00] Executed 'Functions.producer' (Failed, Id=efdce3d9-ba7c-4cdf-aeae-a18aadde68c9)
[2020-08-10 13:58:00] System.Private.CoreLib: Exception while executing function: Functions.producer. Confluent.Kafka: Local: Unknown topic.

Obviously replacing the environment variable with the name of the topic in verbatim works, but I'd expect all config variables to accept environment variables, or at least the same ones as the trigger.

[TsuyoshiUshio]

I believe this PR solves. :) #153

[meken]

@TsuyoshiUshio it looks like that the PR addresses the environment variable resolution only for the Listener (for the @KafkaTrigger), however I'm having trouble with the output binding (@KafkaOutput).

Just had a quick look at the KafkaProducerFactory.cs, the name resolution for other attributes takes place when a ProducerConfig is created. However, this class doesn't contain the topic information, so it should've been resolved somewhere else (in KafkaProducerEntity.cs or maybe earlier KafkaAttributeBinding.cs?).

[TsuyoshiUshio]

Done implementation, could you wait for the next release?