Prometheus-mysql-exporter

Enhancements:
- [[17](https://github.com/braedon/prometheus-mysql-exporter/issues/17)] Add support for scheduling queries with cron expressions (thanks svenXY)

Fixes:
- [[16](https://github.com/braedon/prometheus-mysql-exporter/issues/16)] Fix issue with breaking changes in DBUtils dependency (thanks cdudarchiktw)

Fixes:
- Fix issue where DB updates aren't reflected in metrics.

Enhancements:
- Add [helm chart](https://braedon.github.io/helm/prometheus-mysql-exporter/)
- Add support for Python 3.8
- Move to pure-python MySQL client library (`libmysqlclient-dev` no longer required)

Fixes:
- [[15](https://github.com/braedon/prometheus-mysql-exporter/pull/15)] Try and re-connect when the DB connection fails (thanks liuhuiping2013)
- Fix issue where a query metric going missing would cause an error

Breaking Changes:
- [[9](https://github.com/braedon/prometheus-mysql-exporter/pull/9)] Rework how databases to query are configured

Enhancements:
- Support configuring how metrics are handled on error and when metrics go missing
- [[12](https://github.com/braedon/prometheus-mysql-exporter/pull/12)] Support configuring the timezone to use for MySQL time queries (thanks mhubig)

Breaking Changes:
- Default port changed from 8080 to 9207, [see here for why](https://github.com/prometheus/prometheus/wiki/Default-port-allocations)

Enhancements:
- Add SIGTERM handler
- Optimise Dockerfile
- Add JSON structured logging support
- Allow setting arbitrary logging levels
- Add the --config-dir option

Fixes:
- Fix memory leak in docker image