[DOC] Add task cancellation stats

[sgup432]

What do you want to do?

• Request a change to existing documentation
• Add new documentation
• Report a technical problem with the documentation
• Other

Tell us about your request. Provide a summary of the request.

We had exposed a new stats called TaskCancellationStats(back in OS 2.9) but seems like documentation is still pending. We will have to update existing NodeStats documentation(link) to include this information.

*Version: List the OpenSearch version to which this issue applies, e.g. 2.14, 2.12--2.14, or all.
Applies to all versions>=OS 2.9
What other resources are available? Provide links to related issues, POCs, steps for testing, etc.