Collect error metrics

[qdeslandes]

Generated BPF programs call kfuncs and BPF helpers for various purposes, and those call might fail. In this situation, the program will return with a default verdict. There is currently no way to know if such an error occurred during the program's lifetime, except for reading /sys/kernel/debug/tracing/trace_pipe if bpfilter is running in debug mode (--debug).

An error/failure counter would bring more visibility to this situation and help to understand the program's behavior.