Guard ZipkinTracingAutoConfiguration against missing Encoding bean

[philwebb]

As noted by @onobc we have condition here but not here

[snicoll]

Shoulnd't ZipkinTracingAutoConfiguration have @AutoConfiguration(afterName = "org.springframework.boot.zipkin.autoconfigure.ZipkinAutoConfiguration") as well? Besides the condition, I mean.

[snicoll]

Looking at the smoke test that's failing, and the inconsistent condition, let's go ahead with the above.

[wilkinsona]

Leaving the meeting label on for now as Phil wanted to "double check in our next team meeting that the class is in the correct place. I think it is since the tracing module configures tracing for a bunch of tech and you can use Zipkin without Micrometer tracing".