Ignore unobserved task exceptions explicitly marked as observed

[i3arnon]

Fixes #1026

This allows users to ignore specific exceptions (for example, from third party libraries) by subscribing to UnobservedTaskException and marking the exceptions as observed via UnobservedTaskExceptionEventArgs.SetObserved.

Without this users need to remove the entire UnobservedExceptionTelemetryModule in order to ignore floods of uninteresting exceptions outside their control.

• I ran Unit Tests locally.

[SergeyKanzhelev]

Can you please create an issue with the description of the change and the use example. It will work as a documentation for the feature on GitHub,

Also - update CHANGELOG.md with the reference to that issue.

Thank you for contribution!

[msftclas]

All CLA requirements met.

[i3arnon]

@SergeyKanzhelev ping?

[cijothomas]

@SergeyKanzhelev Can we merge this?

[SergeyKanzhelev]

@cijothomas I left a comment. Please decide. Good feature, I would recommend to enhance it with the config setting to restore the original behavior.

[cijothomas]

@i3arnon Can you make a public bool field on the module, and Ignore the exception based on its value?
If you are not actively working on this, please let us know - I will merge this, as it is for now, and add the public bool field as a followup PR.

Thanks for your contribution!