Thunder Client - Rest API client for VS Code, lightweight Postman alternative
strace - Trace system calls and signals. A diagnostic, debugging and instructional userspace utility.
Ping-API - Automated API testing and monitoring.
perf - Perf is a simple app monitoring solution paired with meaningful alerts.
Runscope - Log, monitor and measure your API usage to solve API problems fast.
DTrace - DTrace is a performance analysis and troubleshooting tool for Solaris, Mac OS X and FreeBSD.