pmrep handling of metricsets with missing metrics

[myllynen]

pmrep fails to start if a metricset is missing a metric part of the metricset. This is expected and happens both with regular and derived metrics. --ignore-incompat can be used to ignore at least a missing derived metric. --ignore-unknown does not have the same effect.

Investigate the different scenarios (live vs archive, regular vs derived, fail vs continue) and at least add a note to the man pages about this. Preferably add some test cases and, if needed, adjust the current operation.