proxy cache display redundancy metrics

[xaleeks]

The proxy cache project should display metrics related to amount of bandwidth saved as a result of having cached the image. This gives us an idea of its usage and how often the image is updated on the upstream. For this, we need to

1. track whether each pull request ends up serving from the cache or the upstream
2. track the requests at the image blob level and not just by manifests
3. calculate something to the effect of size of image (comprising of the image blobs) * requests served from local cache for all unique digests
4. display a running aggregate of this statistic for each proxy project

[github-actions]

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