Charles Proxy - HTTP proxy / HTTP monitor / Reverse Proxy
Zabbix - Track, record, alert and visualize performance and availability of IT resources
Wireshark - Wireshark is a network protocol analyzer for Unix and Windows. It lets you capture and interactively browse the traffic running on a computer network.
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.
Postman - The Collaboration Platform for API Development
Proxyman.io - Proxyman is a high-performance macOS app, which enables developers to view HTTP/HTTPS requests from apps and domains.