fix(NODE-6171): Ensure that rtt is correctly measured

[W-A-James]

Description

What is changing?

Is there new documentation needed for these changes?

No

What is the motivation for this change?

NODE-6171

Release Highlight

Server Selection performance regression due to incorrect rtt measurement

Starting in version 6.6.0, when using the stream server monitoring mode, heartbeats were incorrectly timed as having a duration of 0, leading to server selection viewing each server as equally desirable for selection.

Double check the following

• [X] Ran npm run check:lint script
• [X] Self-review completed using the steps outlined here
• [X] PR title follows the correct format: type(NODE-xxxx)[!]: description
  • Example: feat(NODE-1234)!: rewriting everything in coffeescript
• [X] Changes are covered by tests
• [ ] New TODOs have a related JIRA ticket