Fix Heartbeat Interval

[TimothyMothra]

Fixes issue of heartbeat interval not applying immediately when changed.
#1298

Changes

• setting the heartbeat interval will now reset the timer.

Checklist

• I ran Unit Tests locally.
• CHANGELOG.md updated with one line description of the fix, and a link to the original issue if available.

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• Design discussion issue #
• Changes in public surface reviewed

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[cijothomas]

any way to unit test this change?

[TimothyMothra]

I'm thinking about this now. I'm reviewing our existing unit test coverage. Unit tests are passing locally, but I wanted this to run on our build server to confirm that all our integration tests still pass.

[TimothyMothra]

The System.Threading.Timer is not easily testable. This timer doesn't expose the Interval property so can't inspect that it was changed.

We don't have proper test coverage of how heartbeats are created.
I think we would need to do some significant refactoring of this class so we can manually invoke a "timer event" and validate the telemetry that gets created.
I think that's out of scope for this PR.

I propose that we take these changes as-is and revisit this later.