export: Make it easier to use `export.match` for complex metrics.

Currently histogram matching has to match all three metrics (_bucket, _sum, _count). Summary is similar (_sum _count).

This means customers have to do something like `_.+` regex or all 3 metrics. We could make it easier and match full histogram (all three metrics) once any of the histogram partial metrics are matched (or at least hav WARN log line). Similar for summary.

The problem stays the same with native histograms.