Add custom samplers section to Go sampling documentation

[asapple]

Description

This PR addresses issue #7558 by enhancing the Go sampling documentation with comprehensive information about custom samplers.

Summary of Changes

• Added a new "Custom Samplers" section to the Go sampling documentation
• Provided detailed instructions on how to create custom samplers by implementing the Sampler interface
• Included a complete code example demonstrating how to create a custom sampler that samples spans based on attribute values
• Explained the ShouldSample method implementation with practical example
• Documented the importance of custom sampling logic for specific requirements

Technical Details

The new section explains how developers can implement their own sampling logic by implementing the Sampler interface, which includes:

• The ShouldSample method that takes SamplingParameters and returns a SamplingResult
• The Description method for identifying the sampler

The example demonstrates creating a custom sampler that only samples spans with specific attribute values (e.g., environment=production), giving users full control over their sampling decision logic.

Related Links

• Closes: [Page Feedback]: Add guidance for custom samplers #7558

These changes improve the documentation by providing practical examples that help users understand how to implement custom sampling strategies in their Go applications with OpenTelemetry.

[linux-foundation-easycla]

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[svrnm]

@asapple please make sure that you sign the CLA, also please review our policy on genai usage: https://github.com/open-telemetry/community/blob/main/policies/genai.md

[vitorvasc]

Hi @asapple!

Did you get a chance to review and sign the CLA yet?