hpcrun: rename variable to avoid confusion
hpcrun: rename variable to avoid confusion

Rename "threshold" variable into "metric_period" to avoid confusion between event's threshold and metric's period.

HPCToolkit/hpctoolkit,HPCToolkit/hpctoolkit,HPCToolkit/hpctoolkit,HPCToolkit/hpctoolkit,HPCToolkit/hpctoolkit,HPCToolkit/hpctoolkit