Skip any empty range/boundary elements, not only nil ones.
Don't assume delta has at least one sample per vector element.
Revert "Revert "Ensure that all extracted samples are added to view.""

This reverts commit 4b30fb86b41729e0a3c2b289534fb007abf57b33.
Fix reversed disk flush cutoff behavior.
Fix handling of negative deltas for non-counter values.
Prepend "exporter_" to labels that already exist in exported metrics.

If the metrics exported by a process already contain any of a target's
base labels (such as "job" or "instance", but also any manually assigned
target-group label), don't overwrite that label, but instead add a new
label consisting of the original label name prepended with "exporter_".
This is to accomodate intermediate exporter jobs, which might indicate
e.g. the jobs and instances for which they are exporting data.
Conditionalize disk initializations.

This commit conditionalizes the creation of the diskFrontier and
seriesFrontier along with the iterator such that they are provisioned
once something is actually required from disk.