code: public static boolean isTraceEnabled(StaplerRequest req) {
          if (TRACE)
              return true;
  
          if (TRACE_PER_REQUEST && "true".equals(req.getHeader("X-Stapler-Trace")))
              return true;
  
          return false;
      }
docs: Checks if tracing is enabled for the given request. Tracing can be
  enabled globally with the "stapler.trace=true" system property. Tracing
  can be enabled per-request by setting "stapler.trace.per-request=true"
  and sending an "X-Stapler-Trace" header set to "true" with the request.