Adds function to compute monotonic time.

[d-loret]

Summary: Adds function to get a time reference from the monotonic clock so that elapsed time can be computed.

Test Plan: Ran jsd_egd_test on the Playground testbed and forced some velocity tracking errors. Behavior was equivalent to before the change.

Reviewers: alex-brinkman

[d-loret]

This implementation looks fine to me. I approve the code changes.

Before we make a new release for this, can you confirm you performed a test on hardware?

Confirmed. Tested on Playground testbed.