The `kafka` sink now supports metrics

The kafka sink now supports metrics, making it possible to send metric events through Kafka. Metrics events are encoded into a format that mimics our [internal metrics data model], ideal for custom consumers on the other end. Getting started is easy:

sources:
  host_metrics:
    type: "host_metrics"

sinks:
  kafka:
    type: "kafka"
    inputs: ["host_metrics"]
    encoding:
      codec: "json"

Caveats

We currently do not support ingesting metric events in the kafka source. This is due to the hesitation to introduce yet another metrics format into the world. Instead, we are working to support an open metrics format that the kafka source and sink will support. See issue 5809 for more info.