docs(go): add middleware documentation and examples

[DEVMANISHOFFL]

Description

Added go/MIDDLEWARE.md to document how to use Middleware in the Go SDK.

This guide includes:

• Explanation of the StreamingFunc wrapper pattern.
• Code samples for common use cases:
  • Logging: detailed request/response logging.
  • Authentication: context-based user validation.

Issues Resolved

Fixes #2822

Checklist (if applicable):

• PR title is following https://www.conventionalcommits.org/en/v1.0.0/
• Docs updated

[gemini-code-assist]

Summary of Changes

Hello @DEVMANISHOFFL, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request significantly improves the Go SDK by introducing detailed documentation for middleware implementation, making it easier for developers to add cross-cutting concerns like logging and authentication to their flows. Concurrently, it enhances the GenkitError type to support standard Go error wrapping, providing more robust error handling capabilities. Finally, telemetry data export is optimized to focus solely on Genkit-specific spans, streamlining monitoring and debugging efforts.

Highlights

• New Go Middleware Documentation: A new guide, go/MIDDLEWARE.md, has been added to comprehensively document how to implement and use middleware in the Go SDK, including the StreamingFunc pattern and practical examples for logging and authentication.
• Enhanced Error Wrapping in GenkitError: The GenkitError type in go/core/error.go now supports error wrapping by including an originalError field and implementing the Unwrap() method, improving compatibility with Go's standard error inspection functions like errors.Is and errors.As.
• Telemetry Span Filtering: Telemetry export has been refined in go/core/tracing/tracing.go to include a filteringExporter, ensuring that only spans specifically related to Genkit (those with 'genkit' prefixed attributes) are exported, reducing noise in telemetry data.

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[gemini-code-assist]

Code Review

This pull request introduces documentation for Go middleware, which is a great addition for developers using the Go SDK. The changes also include an important improvement to error handling by enabling error wrapping, making it compatible with errors.Is and errors.As. Additionally, telemetry tracing is refined to filter for Genkit-specific spans, reducing noise. My review includes a few minor suggestions for the documentation and a small performance improvement in the tracing code.

[gemini-code-assist]

The fmt package is imported but not used in this code example. Please remove this line to keep the code clean.

[gemini-code-assist]

The genkit package is imported but not used in this code example. Please remove this line to keep the code clean.

[gemini-code-assist]

It's a standard convention to end files with a newline character. Please add one here.

Suggested change

	}
	}

[gemini-code-assist]

There appears to be a typo with a trailing 'c' at the end of this line.

Suggested change

	- [Middleware Guide](MIDDLEWARE.md): Learn how to add logging, authentication, and custom logic to your flows.c
	- [Middleware Guide](MIDDLEWARE.md): Learn how to add logging, authentication, and custom logic to your flows.

[gemini-code-assist]

To improve performance and avoid potential reallocations inside the loop, it's a good practice to pre-allocate the slice with a known capacity. Since the maximum size of genkitSpans is len(spans), you can initialize it with that capacity.

Suggested change

	var genkitSpans []sdktrace.ReadOnlySpan
	genkitSpans := make([]sdktrace.ReadOnlySpan, 0, len(spans))