protected void onInvoking(Request request, Processor next) {
        if (Trace.Level.Step.includes(trace.getTraceLevel()) || trace.getForceTimestamps()) {
            int traceAt = trace.getForceTimestamps() ? 1 : trace.getTraceLevel();
            trace.trace("Invoke " + next, traceAt);
        }
        if (Trace.Level.Dependencies.includes(trace.getTraceLevel()))
            trace.trace(next.getId() + " " + next.getDependencies().toString(), trace.getTraceLevel());
    }
A hook called when a processor is to be invoked. Overriding methods must call super.onInvoking
container-core/src/main/java/com/yahoo/processing/execution/Execution.java