Application Metrics for Swift can monitor LIVE Swift applications.

Application Metrics for Swift, provides instant and powerful insight into the live performance of your server side Swift applications.

Apple provides macOS Tools to debug Swift application issues

There are several graphical applications and command-line tools available for gathering performance metrics. Some performance tools are installed with Xcode.

LLDB on Linux may be used to debug Swift application issues.

The Swift.org community makes use of the LLDB debugger to provide a rich REPL as well as the debugging environment for the Swift Language. Swift is tightly coupled to the version of the Swift compiler embedded in the debugger. Tight integration of compiler and debugger enables accurate inspection of Swift types as well as full-featured expression evaluation in the context of a rapidly evolving language.