cache: capture metrics related to cache records and pruning #2083

Job	Run time
prepare	1s
test / prepare	1m 56s
test / run (containerd, ./frontend/dockerfile, integration)	4m 16s
test / run (containerd, ./frontend/dockerfile, dockerfile)	5m 45s
test / run (containerd-rootless, ./frontend/dockerfile, integration)	4m 27s
test / run (containerd-rootless, ./frontend/dockerfile, dockerfile)	5m 28s
test / run (containerd-1.6, ./frontend/dockerfile, integration)	4m 11s
test / run (containerd-1.6, ./frontend/dockerfile, dockerfile)	5m 30s
test / run (containerd-snapshotter-stargz, ./frontend/dockerfile, integration)	4m 24s
test / run (containerd-snapshotter-stargz, ./frontend/dockerfile, dockerfile)	5m 51s
test / run (oci, ./frontend/dockerfile, integration)	3m 52s
test / run (oci, ./frontend/dockerfile, dockerfile)	5m 1s
test / run (oci-rootless, ./frontend/dockerfile, integration)	4m 9s
test / run (oci-rootless, ./frontend/dockerfile, dockerfile)	4m 51s
test / run (oci-snapshotter-stargz, ./frontend/dockerfile, integration)	3m 52s
test / run (oci-snapshotter-stargz, ./frontend/dockerfile, dockerfile)	5m 4s
image (mainline)	4m 57s
image (labs)	4m 41s
release	0s
	1h 18m 16s

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