Make Loki settings optional

[andrey-am]

I have created a chart for the exporter, but I am unable to set the "loki_host" parameter to None due to a known bug in Helm (helm/helm#11440). I'm using this chart as a nested subchart.

In my particular use case, I do not require the use of Loki and therefore would like to avoid having to include the "loki_host" and "loki_port" settings in the configuration.