Find metrics for evaluating variant effectiveness.

def _variant_eval_metrics(self, dup_bam):
        """Find metrics for evaluating variant effectiveness.
        """
        base, ext = os.path.splitext(dup_bam)
        end_strip = "-dup"
        base = base[:-len(end_strip)] if base.endswith(end_strip) else base
        mfiles = glob.glob("%s*eval_metrics" % base)
        if len(mfiles) > 0:
            with open(mfiles[0]) as in_handle:
                # pull the metrics as JSON from the last line in the file
                for line in in_handle:
                    pass
                metrics = json.loads(line)
            return metrics
        else:
            return None