public static void appendElapsed(StringBuilder buf, long startTimeNanos, long endTimeNanos) {
        appendElapsed(buf, endTimeNanos - startTimeNanos);
    }
Appends the human-readable representation of the duration between the specified {@code startTimeNanos}
and {@code endTimeNanos} to the specified {@link StringBuilder}.
core/src/main/java/com/linecorp/armeria/common/util/TextFormatter.java