Zabbix - Track, record, alert and visualize performance and availability of IT resources
HttpMaster - HttpMaster is a professional software tool for testing and debugging HTTP applications, primarily aimed at REST API applications and web services.
Datadog - See metrics from all of your apps, tools & services in one place with Datadog's cloud monitoring as a service solution. Try it for free.
Hoppscotch - Open source API development ecosystem
Prometheus - An open-source systems monitoring and alerting toolkit.
MockServer - Easy mocking of any system you integrate with via HTTP or HTTPS.