[SPARK-22135][MESOS] metrics in spark-dispatcher not being registered properly

## What changes were proposed in this pull request?

Fix a trivial bug with how metrics are registered in the mesos dispatcher. Bug resulted in creating a new registry each time the metricRegistry() method was called.

## How was this patch tested?

Verified manually on local mesos setup

Author: Paul Mackles <pmackles@adobe.com>

Closes #19358 from pmackles/SPARK-22135.