Add `seen_current` to kv #4397

Job	Run time
check docs (ubuntu-latest / stable)	54s
check format (ubuntu-latest / nightly)	14s
check linter (ubuntu-latest / stable)	1m 23s
check feature flags (ubuntu-latest / stable)	8m 28s
test (ubuntu-latest / stable)	7m 2s
check examples (ubuntu-latest / stable)	41s
check licenses (ubuntu-latest / stable)	22s
test (macos-latest / stable)	8m 17s
check minimal versions of transitive dependencies (ubuntu-latest / stable)	26s
test (windows-latest / stable)	11m 33s
check minimal supported rust version (ubuntu-latest / msrv)	18s
check spelling (ubuntu-latest / nightly)	23s
	40m 1s

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