@UnstableApi
    public static void appendEpochAndElapsed(StringBuilder buf, long epochMicros, long elapsedNanos) {
        buf.append(dateTimeMicrosecondFormatter.format(getInstantFromMicros(epochMicros))).append('[');
        appendElapsed(buf, elapsedNanos);
        buf.append(']');
    }
Formats the given epoch time in microseconds and duration in nanos to the format
"epochMicros[elapsedNanos]" and appends it to the specified {@link StringBuilder}.
This may be useful to record high-resolution timings such as {@link ClientConnectionTimings}.
core/src/main/java/com/linecorp/armeria/common/util/TextFormatter.java