cache: capture metrics related to cache records and pruning #2369

Job	Run time
prepare	7s
binaries (darwin/amd64)	1m 15s
binaries (darwin/arm64)	1m 12s
binaries (linux/amd64)	2m 22s
binaries (linux/arm/v7)	2m 16s
binaries (linux/arm64)	2m 5s
binaries (linux/s390x)	2m 2s
binaries (linux/ppc64le)	2m 35s
binaries (linux/riscv64)	1m 58s
binaries (windows/amd64)	1m 16s
binaries (windows/arm64)	1m 14s
test / prepare	2m 1s
test / run (containerd, ./client ./cmd/buildctl ./worker/containerd ./solver ./frontend, integration)	7m 20s
test / run (containerd-rootless, ./client ./cmd/buildctl ./worker/containerd ./solver ./frontend, integr...	8m 0s
test / run (containerd-1.6, ./client ./cmd/buildctl ./worker/containerd ./solver ./frontend, integration)	7m 9s
test / run (containerd-snapshotter-stargz, ./client ./cmd/buildctl ./worker/containerd ./solver ./fronte...	8m 0s
test / run (oci, ./client ./cmd/buildctl ./worker/containerd ./solver ./frontend, integration)	5m 19s
test / run (oci-rootless, ./client ./cmd/buildctl ./worker/containerd ./solver ./frontend, integration)	5m 17s
test / run (oci-snapshotter-stargz, ./client ./cmd/buildctl ./worker/containerd ./solver ./frontend, int...	5m 1s
test / run (./..., 1, integration gateway)	4m 6s
test / run (./client, containerd, nydus, integration)	7m 34s
test / run (./client, oci, nydus, integration)	4m 58s
test / run (./..., nydus, 1, integration)	4m 22s
image	0s
release	0s
	1h 27m 29s

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