Support pgbouncer 1.24 reserve pool change

[keithf4]

Describe the issue:
pgbouncer 1.24 changed the name of the reserve_pool column to reserve_pool_size

Tell us about your environment:

• pgMonitor version: 5.2
• Container or non-container:
• Container name / image:
• Operating System for non-container:
• PostgreSQL Version:
• Exporter(s) in use (incl. version):
• Prometheus version:
• AlertManager version:
• Grafana version: