The ExecuteCoreAsync method is now Outcome based

[martintmk]

Details on the issue fix or feature implementation

This change simplifies the implementation of strategies and improves performance in cases when there are many exceptions thrown (i.e. circuit breaker is opened).

The perf impact of this change is non-trivial. It comes from wrapping the user-callback and converting it to outcome-based one. If caller provides the outcome-based compatible callback the impact is minimal (demonstrated in the ResilienceStrategyBenchmark).

Benefits:

• Simplify the implementation of strategies (do not have to deal with exceptions).
• Allow to attach additional information to the Outcome in the future.
• Improves the perf in error cases as no-exception are thrown (for example when circuit is opened). Note that the caller needs to use the Outcome-based API to take advantage of this. By default, the perf will be around the same if the normal Execute methods are used.

Downsides:

• Perf hit for success cases.

Benchmark for error cases:

Method	Mean	Error	StdDev	Ratio	RatioSD	Gen0	Allocated	Alloc Ratio
ExecuteAsync_Exception_V7	29,569.5 ns	70.74 ns	101.46 ns	91.72	1.72	0.0916	2888 B	12.89
ExecuteAsync_Exception_V8	31,845.8 ns	158.22 ns	221.81 ns	98.72	1.85	0.0610	2824 B	12.61
ExecuteAsync_Outcome_V8	322.5 ns	4.09 ns	5.87 ns	1.00	0.00	0.0086	224 B	1.00

Confirm the following

• I started this PR by branching from the head of the default branch
• I have targeted the PR to merge into the default branch
• I have included unit tests for the issue/feature
• I have successfully run a local build

[martincostello]

Nice.

[geeknoid]

I'm not crazy about the perf hit, but I think the improvements are warranted.

[martintmk]

Ok, I'll go ahead, polish this and prepare for merging.

[codecov-commenter]

Codecov Report

Merging #1195 (41074c3) into main (71b760a) will increase coverage by 0.39%.
The diff coverage is 100.00%.

❗ Current head 41074c3 differs from pull request most recent head d8d0558. Consider uploading reports for the commit d8d0558 to get more accurate results

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@@            Coverage Diff             @@
##             main    #1195      +/-   ##
==========================================
+ Coverage   83.31%   83.70%   +0.39%     
==========================================
  Files         277      278       +1     
  Lines        6286     6439     +153     
  Branches     1023     1023              
==========================================
+ Hits         5237     5390     +153     
  Misses        844      844              
  Partials      205      205              

Flag	Coverage Δ
linux	?
macos	83.70% <100.00%> (+0.39%)	⬆️
windows	83.70% <100.00%> (+0.39%)	⬆️

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Impacted Files	Coverage Δ
...Core/Hedging/Controller/HedgingExecutionContext.cs	100.00% <ø> (ø)
src/Polly.Core/NullResilienceStrategy.cs	100.00% <ø> (ø)
.../Polly.Core/Strategy/ResilienceStrategyPipeline.cs	100.00% <ø> (ø)
...CircuitBreaker/CircuitBreakerResilienceStrategy.cs	100.00% <100.00%> (ø)
...ircuitBreaker/Controller/CircuitStateController.cs	100.00% <100.00%> (ø)
.../Polly.Core/Fallback/FallbackResilienceStrategy.cs	100.00% <100.00%> (ø)
src/Polly.Core/Hedging/Controller/TaskExecution.cs	100.00% <100.00%> (ø)
...rc/Polly.Core/Hedging/HedgingResilienceStrategy.cs	100.00% <100.00%> (ø)
...c/Polly.Core/ResilienceStrategy.Async.ValueTask.cs	100.00% <100.00%> (ø)
.../Polly.Core/ResilienceStrategy.Async.ValueTaskT.cs	100.00% <100.00%> (ø)
... and 10 more

[geeknoid]

Given this new property, then you should change the Exception property to just

public Exception? => ExceptionDispatchInfo?.SourceException;

which would shrink the Outcome and Outcome<T> structs by 1/3, speeding up all that struct copying.