Prometheus-pgbouncer-exporter

- [BREAKING CHANGE] Renamed `pgbouncer_stats_queries_total` to `pgbouncer_stats_requests_total` on pgbouncer <= 1.7
- [FEATURE] Added pgbouncer >= 1.8 support [8](https://github.com/spreaker/prometheus-pgbouncer-exporter/pull/8) (thanks to [bitglue](https://github.com/bitglue)), including the following new metrics:
- `pgbouncer_stats_transactions_total`
- `pgbouncer_stats_queries_total`
- `pgbouncer_stats_transactions_duration_microseconds`
- `pgbouncer_stats_waiting_duration_microseconds`

- [BUGFIX] Fixed `PGBOUNCER_EXPORTER_PORT` environment variable data type [7](https://github.com/spreaker/prometheus-pgbouncer-exporter/pull/7)

- [FEATURE] Added `pgbouncer_databases_database_pool_size`, `pgbouncer_databases_database_reserve_pool_size` and `pgbouncer_databases_database_current_connections` metrics [3](https://github.com/spreaker/prometheus-pgbouncer-exporter/pull/3)
- [BUGFIX] Fixed `PyYAML` dependency declaration [4](https://github.com/spreaker/prometheus-pgbouncer-exporter/pull/4)
- [BUGFIX] Ensure the exporter immediately exit on SIGTERM

- [1](https://github.com/spreaker/prometheus-pgbouncer-exporter/pull/1) - Log connection or scrape failures as errors, not debug

- Added config file validation
- Added `--log-file` cli argument support and handle of `kill -HUP` to re-open the log file description (useful in combination with logrotate)

- Ensure the `prometheus-pgbouncer-exporter` package is installed on Python >= 3