fix: introduce mutex to avoid nil deferences for reloads while scraping

[burningalchemist]

Fixes #970

This pull request introduces thread-safety improvements to the exporter and target types by adding mutex locks to protect shared resources. These changes ensure that concurrent access to critical sections, such as target and connection management, is properly synchronized, preventing race conditions and potential data corruption.

Thread-safety improvements:

• Added a sync.RWMutex (mu) to the exporter struct and wrapped accesses to the targets slice in read/write locks to prevent concurrent read/write issues. (exporter.go, [1] [2] [3]
• Added a sync.RWMutex (mu) to the target struct and used locks in Close and ping methods to ensure only one goroutine can modify or close the database connection at a time. (target.go, [1] [2] [3]