filterx: `update_metrics()` and `flatten()` perf improvements #1067

Job	Run time
create-source-tarball	1m 51s
create-packages / almalinux-8	3m 45s
create-packages / almalinux-9	6m 52s
create-packages / debian-bookworm	6m 20s
create-packages / debian-testing	6m 14s
create-packages / fedora-41	7m 38s
create-packages / ubuntu-noble	8m 21s
	41m 1s

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