Awesome that you're interested in contributing to Apache Beam! If you're specifically interested in the Go SDK, the task you identified is a good one to start with. I assigned it to you. I also added a few similar tasks listed below as alternatives. Feel free to pick the one you prefer and re-assign as appropriate (or I can do it for you). It's best that the JIRAs are assigned before any work is done so avoid accidental duplication.
BEAM-4466 Add Go TF-IDF example
BEAM-4467 Add Go Autocomplete example
The main caveat for Go streaming pipelines is that they currently only really work on Dataflow, because the only streaming IO connector is PubSub and the direct runner supports batch only. In the near future, however, the ULR and Flink runner will support portable streaming pipelines, including Go. If it is too impractical to work with the IO used by corresponding Java/Python examples, feel free to deviate by using textio or similar instead. There may also be incomplete feature work in Go that prevents an direct translation.
Please feel to ask questions in the JIRAs or on the dev list. Happy to help!