refactor(multi): simplify API with functional options pattern

[erka]

This PR

• Add WithProvider option for registering named providers with optional hooks
• Remove ProviderMap type in favor of variadic functional options
• Change NamedProvider from struct to interface
• Update NewProvider signature to take strategy first, then options
• Add Track method support to MultiProvider for tracking events
• Update tests, documentation and examples to reflect new API

[codecov]

Codecov Report

❌ Patch coverage is 90.57971% with 13 lines in your changes missing coverage. Please review.
✅ Project coverage is 83.07%. Comparing base (7fd7ab6) to head (08f4ade).

Files with missing lines	Patch %	Lines
openfeature/multi/multiprovider.go	91.45%	7 Missing and 3 partials ⚠️
openfeature/multi/isolation.go	57.14%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #444      +/-   ##
==========================================
+ Coverage   82.89%   83.07%   +0.17%     
==========================================
  Files          27       27              
  Lines        2327     1985     -342     
==========================================
- Hits         1929     1649     -280     
+ Misses        363      299      -64     
- Partials       35       37       +2     

Flag	Coverage Δ
e2e	83.07% <90.57%> (+0.17%)	⬆️
unit	83.07% <90.57%> (+0.17%)	⬆️

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[erka]

/gemini review

[gemini-code-assist]

Code Review

This pull request implements the Tracker interface for the multi-provider, enabling the forwarding of tracking calls to all ready providers that support tracking. The changes include adding the Tracker interface implementation to the Provider struct, modifying the NewProvider function to handle provider hooks correctly, updating the event handling logic in the Init function, and implementing the Track function to forward tracking calls. Additionally, the pull request includes corresponding unit tests to verify the functionality of the Tracker implementation.

[erka]

/gemini review

[gemini-code-assist]

Code Review

This pull request introduces a functional options pattern to simplify the API of the multi-provider, replaces the ProviderMap type with variadic functional options, changes NamedProvider from a struct to an interface, updates the NewProvider signature, adds Track method support, and updates tests and documentation. The changes enhance the flexibility and maintainability of the multi-provider. I have provided comments to address potential issues related to error handling, interface usage, and code clarity.

[toddbaert]

I think @thisthat was just mentioning the other day we should do this.

[toddbaert]

I have no issues with the changes but I think the title should be feat(multi): add tracking, functional options or something like that, since we added that new functionality.

Also it's probably worth noting there's breaking changes here with the multi-provider, but I think that's fine since we explicitly said it's experimental, and it's an auxiliary feature (so let's not add a ! to the title).

[toddbaert]

@erka I'm sorry, I during review I accidentally removed your changes, and GH auto-closes PRs when that happens.

I opened a new one here with all your commits. We can either continue there, or you can try to open this PR again if you move the changes back to this branch (I think that works).