fix: use modern golang functions (go fix)

[burningalchemist]

This pull request introduces several improvements and code modernizations across the codebase, focusing on simplifying code, improving type usage, and leveraging newer Go standard library features. The main areas affected are utility functions, type definitions, duplicate label checking logic, and goroutine management.

Code Modernization and Type Improvements:

• Replaced the use of interface{} with any in the LogFunc type and its method to align with modern Go conventions, making the code more idiomatic.

Logic Simplification and Use of Standard Library:

• Updated duplicate label checking in MetricConfig.validateKeyLabels to use slices.Contains instead of a manual loop, simplifying the logic and improving readability. [1] [2]

Utility and Concurrency Enhancements:

• Changed the splitting of the Accept-Encoding header to use strings.SplitSeq (likely a custom or newer function) and fixed the iteration method for improved correctness and clarity in decorateWriter.
• Modernized goroutine management in the Warmup.Run method by replacing manual sync.WaitGroup management with the more concise cwg.Go pattern, reducing boilerplate and potential errors.