loki(): Storing messages in a Grafana Loki database

In syslog-ng OSE 4.4 and later versions the `loki()` destination can be used to send log data to Grafana Loki. For more information on the message format, see Grafna Loki HTTP endpoint.

Example: loki() destination configuration

loki(
    url("localhost:9096")
    labels(
        "app" => "$PROGRAM",
        "host" => "$HOST",
    )

    workers(16)
    batch-timeout(10000)
    batch-lines(1000)
);