fix: export scrape_errors_total for related job filters

[burningalchemist]

resolves #903

This pull request introduces improved job filter handling and error reporting in the SQL exporter, as well as several code quality enhancements. The main changes include making job filter validation stricter, filtering metrics based on job filters, and updating logging and utility functions for clarity and robustness.

Job filter handling and error reporting:

• The SetJobFilters method now validates job names, returns errors for invalid filters, and is updated to return an error instead of silently failing. The HTTP handler responds with a 400 error if invalid filters are provided. [1] [2]
• In single target mode, job filters are ignored, and appropriate warnings are logged.

Metrics filtering:

• Added the FilterScrapeErrorsTotal method to the Exporter interface and implementation, which filters the scrape_errors_total metric family to only include metrics for jobs specified in the job filters. This is now applied in the HTTP handler. [1] [2] [3]

Logging and code quality improvements:

• Improved logging statements for better readability and context, especially in the caching collector and error scenarios. [1] [2] [3] [4]
• Refactored utility functions: parseContextLog now handles invalid items more gracefully and uses strings.SplitSeq; TrimMissingCtx simplified to use strings.TrimPrefix. [1] [2]

General refactoring:

• Updated function signatures for clarity and consistency, including changes to how targets and jobs are constructed and initialized. [1] [2] [3]