public static void appendEpochMillis(StringBuilder buf, long timeMillis) {
        buf.append(dateTimeFormatter.format(Instant.ofEpochMilli(timeMillis)))
           .append('(').append(timeMillis).append(')');
    }
Formats the given epoch time in milliseconds to typical human-readable format
"yyyy-MM-dd'T'HH_mm:ss.SSSX" and appends it to the specified {@link StringBuilder}.
core/src/main/java/com/linecorp/armeria/common/util/TextFormatter.java