Avoid empty intervals from smoothing below interval_min

[tilsche]

Currently, flex requests with smoothing with a interval_max close to the regular update interval can lead to empty intervals (count=0) even with relatively regular updates.

This should be prevented, as count=0 intervals are interpreted by metricq-grafana as "missing data" and emptied to show a gap rather than interpolate with LAST semantic.

The smoothing should instead extend the empty intervals until they contain data up to the length of min_interval.

This is also a trigger for some forms of metricq/metricq-webview#173

[tilsche]

@bmario coould you maybe resurrect https://github.com/metricq/hta/tree/experimental-flex-tests

That would be a foundation for fixing this issue. That we ran into again today :/