Add custom fields to SplunkHttp

[membersound]

I would like to add custom metadata fields.
That could maybe be achieved by adding a @PluginElement("Metadata") final Map<String> metadata or similar.
Then the following could be possible:

        <SplunkHttp
                name="SPLUNK"
                url="..."
                token="..."
                index="it-dev-test"...>
            <PatternLayout pattern="%m"/>
            <Metadata>
                <KeyValuePair key="groupname" value="mygroup"/>
            </Metadata>
        </SplunkHttp>

Please open the appender to take custom fields.