v1.19.0: metrics endpoint, basic auth, start/stop checks

This is a minor release with some useful new features:

• A startup field for checks, and the ability to start and stop checks (#560)
• A new basic type identity for Basic HTTP authentication (#563)
• A metrics endpoint at /v1/metrics in OpenMetrics/Prometheus format, providing metrics for services and checks (#519)

Full Changelog: v1.18.0...v1.19.0