cache: capture metrics related to cache records and pruning #2082

Job	Run time
prepare	1s
test / prepare	1m 44s
test / run (containerd, ./frontend/dockerfile, integration)	4m 34s
test / run (containerd, ./frontend/dockerfile, dockerfile)	6m 3s
test / run (containerd-rootless, ./frontend/dockerfile, integration)	4m 13s
test / run (containerd-rootless, ./frontend/dockerfile, dockerfile)	5m 31s
test / run (containerd-1.6, ./frontend/dockerfile, integration)	4m 55s
test / run (containerd-1.6, ./frontend/dockerfile, dockerfile)	5m 39s
test / run (containerd-snapshotter-stargz, ./frontend/dockerfile, integration)	5m 40s
test / run (containerd-snapshotter-stargz, ./frontend/dockerfile, dockerfile)	6m 0s
test / run (oci, ./frontend/dockerfile, integration)	4m 2s
test / run (oci, ./frontend/dockerfile, dockerfile)	4m 56s
test / run (oci-rootless, ./frontend/dockerfile, integration)	3m 58s
test / run (oci-rootless, ./frontend/dockerfile, dockerfile)	4m 56s
test / run (oci-snapshotter-stargz, ./frontend/dockerfile, integration)	3m 53s
test / run (oci-snapshotter-stargz, ./frontend/dockerfile, dockerfile)	4m 58s
image (mainline)	4m 52s
image (labs)	5m 0s
release	0s
	1h 20m 55s

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