Implement support to the PERF_* variables in libmodsecurity

[zimmerle]

The following variables needs to be implemented inside libModSecurity:

• PERF_ALL
• PERF_COMBINED
• PERF_GC
• PERF_LOGGING
• PERF_PHASE1
• PERF_PHASE2
• PERF_PHASE3
• PERF_PHASE4
• PERF_PHASE5
• PERF_RULES
• PERF_SREAD
• PERF_SWRITE

More information about these variables can be found at ModSecurity reference manual: https://github.com/SpiderLabs/ModSecurity/wiki/Reference-Manual

[zimmerle]

Instead of using the performance variables which can actually have a negative impact on performance, we would suggest the utilization of Linux instrumentation tools. The utilization of stap, for instance, can lead the same results as the Performance variables plus it allows the user to use the existent tools such as flame-charts to plot the performance information in a easy to read way. There is a stap example in the nginx repository: https://github.com/SpiderLabs/ModSecurity-nginx/blob/master/ngx-modsec.stp

Having said that, I am closing this issue.

[dune73]

DURATION can be used to substitute the timings for phase 1 through 5. But the other ones would demand systemtap. DURATION also is buggy and not implemented correctly (if correctness means equal to 2.9.x).

Also the substitution is much more complicated than the native availablity of the perf variables.

I'm going into some length on this problem and the use cases for the performance variables in this ML message: https://sourceforge.net/p/mod-security/mailman/message/36276827/