I found the anser by myself.
instead of a seperate file I putted the config values inside the runtime.cfg and prefixed all values with ‘org.openhab.io.metrics.MetricsRestController:’
like
org.openhab.io.metrics.MetricsRestController:influxMetricsEnabled=true
org.openhab.io.metrics.MetricsRestController:influxURL=http://influxdb:8086
org.openhab.io.metrics.MetricsRestController:influxDB=openhab_metrics
org.openhab.io.metrics.MetricsRestController:influxUsername=****
org.openhab.io.metrics.MetricsRestController:influxPassword=****
org.openhab.io.metrics.MetricsRestController:influxUpdateIntervalInSeconds=300