fix: memory leak on config reloading

[burningalchemist]

resolves #927

This pull request introduces robust resource management for collectors and targets, ensuring that all database connections and prepared statements are properly closed during reloads. This addresses potential memory and goroutine leaks, especially during repeated configuration reloads. It also adds a comprehensive test to detect memory leaks during reload cycles.

Resource management improvements:

• Added a Close() method to the Collector, Target, and Query interfaces/types, ensuring all prepared statements and database connections are released when no longer needed. (collector.go [1] query.go [2] target.go [3]
• Implemented the Close() method for collector and cachingCollector types to release all prepared statements. (collector.go collector.goR89-R107)
• Implemented the Close() method for the target type to close all associated collectors and the underlying database connection pool, collecting and reporting any errors. (target.go target.goR149-R179)

Reload and cleanup logic:

• Updated reload logic to call a new closeTargets() helper, ensuring all old targets' resources are closed before updating to new targets or jobs during a reload. (reload.go [1] [2]
• Added the closeTargets() helper function to close each target and log (but not propagate) errors, preventing a single failure from blocking the reload process. (reload.go reload.goR97-R112)

Testing for leaks:

• Added a new test TestReloadMemoryLeak in reload_test.go that performs multiple reload cycles and checks for goroutine leaks, ensuring that resource cleanup is effective. (reload_test.go reload_test.goR1-R146)