[FEATURE] Add mechanism to determine when background fetcher is done

[Kern--]

Description

The background fetcher is a reliability mechanism to limit the maximum amount of time that the SOCI snapshotter requires a network connection when running a container. Right now, there is no way to determine when the background fetching is complete and there are no metrics to indicate the health of the background fetcher (e.g. number of spans downloaded, number skipped due to errors, whether the fetcher is still running for a specific layer).

It could be helpful to understand the overall health of the snapshotter to know this information.

Describe the solution you'd like

1. Some mechanism to query whether the background fetch has completed and whether it fully downloaded the image.
2. Some set of health metrics that can be collected

Describe any alternative solutions/features you've considered

No response

Any additional context or information about the feature request

No response