`ILoggingEvent#getInstant()` is introduced in version 1.3.
But, the default implementation returns null.
I think this is the compatibility issue between 1.2.x ~ 1.3.x.
I propose default implementation returns `Instant.ofEpochMills(getTimeStamp())` to improve compatibility with 1.2 or prior.

In some cases, the compatibility breaks logging.

Reproduction

In my case, the compatibility is broken in the following conditions.

• use logstash-logback-encoder 5.3
• use armeria-logback 1.23.1
  • this module is compiled with logback 1.2.x
• use logback 1.4.x
• use this logback config

    <configuration>
        <appender name="STDOUT" class="ch.qos.logback.core.ConsoleAppender">
            <encoder class="net.logstash.logback.encoder.LogstashEncoder" />
        </appender>
        <appender name="RCEA" class="com.linecorp.armeria.common.logback.RequestContextExportingAppender">
            <appender-ref ref="STDOUT" />
        </appender>
        <root level="info">
            <appender-ref ref="RCEA" />
        </root>
    </configuration> 

The logstash-logback-encoder 7.3 calls `getInstant()` when logback 1.3 or higher, here].
But, armeria-logback has LoggingEventWrapper that extends logback's LoggingEvent.
The LoggingEventWrapper lacks the implementation of `getInstant()` because it depends on logback 1.2 to keep compatibility.
Therefore, logstash-logback-encoder are going to get `null` by default implementation of `getInstant` in ILoggingEvent.
After that, logging is not working by NullPointerException.