* Export "null" in jsonl instead of NaN
The NaN appeared because of staleness markers that were added for compatibility. I think it's better to use json `null`, implemented here.
Also maybe it also makes sense to add a flag like `?skip-staleness-markers=true` to `/export`, to skip nulls at all?
* Update app/vmselect/prometheus/export.qtpl
* app/vmselect/prometheus/export.qtpl.go: `make quicktemplate-gen`
* docs/CHANGELOG.md: document the change
Co-authored-by: Aliaksandr Valialkin <valyala@victoriametrics.com>