protected final long requestStartTimeNanos() {
        checkState(isRequestStartTimeSet(), "requestStartTime is not set.");
        return requestStartTimeNanos;
    }
Returns the {@link System#nanoTime()} value when the request started.

@throws IllegalStateException if the request start time is unspecified.
core/src/main/java/com/linecorp/armeria/common/AbstractRequestContextBuilder.java