Metric selection support for pre-aggregated metrics

[eswdd]

Typically pre-aggregated metrics will be of the form:

some.metric.
or
some.metric.-

Would be good to offer mechanism to select pre-aggregated metrics and choose the aggregation(s) to display on the graph - likely useful to specify 1 or more metrics and 1 or more functions.

We could filter out pre-aggregated metrics from the main tree but show an indicator that pre-aggregated versions exist (for example where some metrics are both pre-aggregated and not)