simplify the code in updateRecord()

changes:
- each metric handles what writer to use and how to use it
- MetricCollection will fetch a new metric to update record 
- MetricCollection handles how a new metric is returned

When a new metric is added, all we have to do is have the metric declare what writer to use and how to use it. (Like the way we did with RecordFilter and RecordReader)
Same idea here.  If you declare

@Resource
Iterable<Metric<AccessRecord, ?>>

would Spring be able to assemble the collection for you?