filterx: `update_metrics()` and `flatten()` perf improvements #1065

Job	Run time
create-source-tarball	1m 58s
create-packages / almalinux-8	3m 51s
create-packages / almalinux-9	6m 49s
create-packages / debian-bookworm	5m 58s
create-packages / debian-testing	6m 5s
create-packages / fedora-41	7m 4s
create-packages / ubuntu-noble	8m 56s
	40m 41s

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