metricbeat/module/metricbeat: Implement connection reuse with a single long-lived MongoDB client

**Describe the enhancement:** Implement connection reuse with a single long-lived MongoDB client to minimize connection pile-up at lower intervals, eliminating the overhead of constant connection creation and destruction. While the current implementation works adequately for intervals of 30-60+ seconds, this approach will provide significantly better performance and reliability across all interval configurations.

**Describe a specific use case for the enhancement or feature:** It applies to all use-cases.