Add kafka traces receiver

[mapno]

PR Description

Adds the kafka receiver

Which issue(s) this PR fixes

Closes #484

Notes to the Reviewer

PR Checklist

• CHANGELOG updated
• Documentation added
• Tests updated

[rfratto]

LGTM! Before I merge, can you update the changelog?

@joe-elliott Typically we add all the receivers to the install manifest (rendered as YAML here), but since the Kafka receiver works so differently, I don't think we can do the same thing here. WDYT?

[joe-elliott]

Yeah, agree, it doesn't belong in the install manifest. Is there a place that makes sense to document this new option?

[rfratto]

Our docs here are really weak. IMO for now let's just have a list in the docs for tempo_instance_config, something like:

# Receiver configurations are mapped directly into the OpenTelmetry receivers block.
#   At least one receiver is required. Supported receivers: otlp, jaeger, kafka, [...]
#   Documentation for each receiver can be found at https://github.com/open-telemetry/opentelemetry-collector/blob/7d7ae2eb34b5d387627875c498d7f43619f37ee3/receiver/README.md
receivers:

[mapno]

Added the documentation and manually tested the receiver by modifying the docker-compose example. I didn't think it was worth modifying the code. Let me know if you want me to test it some other way (e.g. integration tests).

[rfratto]

Thanks again! I'll merge once the conflicts are resolved.

[mapno]

Thanks again! I'll merge once the conflicts are resolved.

They should be solved now