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.
Primary changes:
* Strictly typed unmarshalling of metric values
* Schema types are contained by the processor (no "type entity002")
Minor changes:
* Added ProcessorFunc type for expressing processors as simple
functions.
* Added non-destructive `Merge` method to `model.LabelSet`
Julius Volz
Julius Volz
Bernerd Schaefer