Add Prometheus as available datasource

[Mikesch-mp]

It is not needed. just use InfluxDB as datasource. The only thing this does when graphite is choosed is to change some special chars

    public static function graphiteReplace($string = "")
    {
        $string = preg_replace('/[^a-zA-Z0-9\*\-:]/', '_', $string);

        return $string;
    }

Just use InfluxDB as the source in the configuration