async-nats: Support `sample_freq` values containing a % #4516

Job	Run time
check format (ubuntu-latest / nightly)	11s
check feature flags (ubuntu-latest / stable)	15m 21s
check docs (ubuntu-latest / stable)	1m 13s
check examples (ubuntu-latest / stable)	51s
check licenses (ubuntu-latest / stable)	21s
check linter (ubuntu-latest / stable)	1m 39s
check minimal versions of transitive dependencies (ubuntu-latest / stable)	29s
check minimal supported rust version (ubuntu-latest / msrv)	18s
check spelling (ubuntu-latest / nightly)	30s
test (ubuntu-latest / stable)	7m 4s
test (macos-latest / stable)	8m 15s
test (windows-latest / stable)	10m 52s
	47m 4s

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