Add System.Threading.PeriodicTimer

[stephentoub]

Fixes #31525

I still wonder whether IAsyncEnumerable<T> would be a better way to expose this, e.g. a static IAsyncEnumerable<TimeSpan> Periodic(TimeSpan period, CancellationToken cancellationToken = default); as a static method on System.Threading.Timer. One concern there was around what data we'd actually return as the T, but I think an elapsed time since start or something along those lines could be reasonable, even if generally ignored. The other concern was around how one stops the enumeration from outside of the consuming loop (the consuming loop itself can just break out and dispose of the enumerator); I'm personally comfortable using the cancellation token for that purpose, but I know there were concerns about modeling such stopping as an exceptional situation. On the balance, I think that model is cleaner, and I like the idea of adding a new method to Timer rather than increasing the number of timer types we have.

cc: @davidfowl, @terrajobst, @kouvel

[dotnet-issue-labeler]

Note regarding the new-api-needs-documentation label:

This serves as a reminder for when your PR is modifying a ref *.cs file and adding/modifying public APIs, to please make sure the API implementation in the src *.cs file is documented with triple slash comments, so the PR reviewers can sign off that change.

[dotnet-issue-labeler]

I couldn't figure out the best area label to add to this PR. If you have write-permissions please help me learn by adding exactly one area label.

[omariom]

I still wonder whether IAsyncEnumerable would be a better way to expose this, e.g. a static IAsyncEnumerable Periodic(TimeSpan period, CancellationToken cancellationToken = default); as a static method on System.Threading.Timer.

👍

The usage of the new timer is not very different from a while loop with a Task.Delay(..) within. It is not very.. innovative.
Less allocations is not obvious, one would need to read about it in the remarks section of the docs. Also Task.Delays are easier to understand.

[stephentoub]

Failures are https://github.com/dotnet/core-eng/issues/13324