ShortenedThrowableConverter: <exclude> and <exclusions> are mutually exclusive

[brenuart]

Suppose the following configuration:

<throwableConverter class="net.logstash.logback.stacktrace.ShortenedThrowableConverter">
    <exclude>pattern1</exclude>
    <exclusions>${STACK_EXCLUSIONS}</exclusions>
    <exclude>pattern2</exclude>
</throwableConverter>

Using the <exclusions> with an environment variable is an easy way to configure additional exclusion patterns in addition to those already configured in the XML file. However, it happens that <exclusions> clears the exclusions already configured and replace them with new values. This sample configuration results in only TWO exclusion patterns with the first one being ignored...

To avoid confusion the <exclusions> keyword should ADD new patterns instead of replacing those already defined.

[brenuart]

@philsttr What do you think of this?

[philsttr]

Sounds good.

I think there is a low probability that anyone is using both.

Worth calling out as a slight backwards incompatibility in the release notes. But I don't think it requires a major version bump, since this seems more like a bugfix.