Support providing "service" and "source" metadata

[rootd00d]

Describe what happened:

• Logs are arriving to Datadog with "source: undefined"
• No ability to mark the name of the service

Describe what you expected: A flexible means of labeling the service and log source in fluentd configuration

[rootd00d]

Apparently, I just need to read the FAQs: https://docs.datadoghq.com/logs/faq/how-to-send-logs-to-datadog-via-external-log-shippers/#fluentd

In the end, we used the record_transformer plugin to do something like this to add the required metadata in Fluentd --

<filter filtered.**>
    @type record_transformer
    <record>
        message ${record["log"]}
        ddsource "${tag_parts[4]}"
        syslog.hostname "${hostname}"
        syslog.appname "${tag_parts[4]}"
    </record>
    remove_keys log
</filter>