Implement metrics for loading state

[bcarter97]

Description

It would be useful to expose some metrics on the state of the Topic loading. To start with I think we should have the following:

• Callback for on record received
• Callback for when loading starts
• Callback for when loading finishes
• Callback for when loading errors

For now this will be a noOp metric, but will be exposed on the TopicLoader interface so people can implement their own. Ideally we could have a kafka-topic-loader-kamon module or similar that implements the metric and exposes values to Kamon.