Optimize Jinja2 processing during metrics export to Prometheus

[andy-maier]

Currently, the Jinja2 expressions for labels in the metric definition file are parsed each time Prometheus retrieves metrics from the exporter.

This should be optimized, e.g. by compiling the Jinja2 expressions so that the Jinja2 parsing is done only once. The evaluation of the Jinja2 expressions cannot be done just once, because it way include other metrics or property values.