Introduce RegexRunnerPool to reuse multiple RegexRunner instances at once under contention

[neon-sunset]

Just a proof of concept for now, will update description if it pans out.

[dotnet-policy-service]

Tagging subscribers to this area: @dotnet/area-system-text-regularexpressions
See info in area-owners.md if you want to be subscribed.

[danmoseley]

is there any value in attmepting to use an existing pool such as something out of Microsoft.Extensions.ObjectPool

[neon-sunset]

Adding a queue/stack pool to existing cached runner in a field essentially mimics DefaultObjectPool<T>: https://github.com/dotnet/aspnetcore/blob/main/src/ObjectPool/src/DefaultObjectPool.cs

However, I didn't see an easy way to bring it over and ObjectPool opts for using ConcurrentQueue<T> instead, which incurs >800B in allocations compared to ConcurrentStack<T> which is 56B to allocate + add one element. I wanted to keep memory usage to a minimum and avoid increasing assembly size more than necessary - regex patterns may still be fairly shortlived even if shared by multiple threads, or there can be thousands of them kept for a long time (which matches the workload of the application I found the initial issue in). This is still just an attempt - it needs a less noisy benchmark and I need to look into test failures first. Thank you for reviewing this!

[danmoseley]

thank you for working on it.

[danmoseley]

if you keep this code you could add some Debug.Assert that it wasn't already returned