Fixes for the helm chart

[rwxd]

Hey, thanks for this tool and the grafana dashboard.

I deployed the exporter with the helm chart in this repository, but had some problems.

• The scraping interval is an integer and not a string.
• The selectorLabels on the ServiceMonitor can be wrong. The variable should be used, like in the other manifests.
• If the config value is not written as a string it can't be converted to base64. But the map can be converted to a yaml string.

[pdreker]

Thanks for your contribution!

The helm chart is kind of a a "hot mess", so any improvements are welcome.