Move metrics folder from nested within exporters folder

[BrandenRice]

This PR updates a few things:

• Move metrics folder from nested within exporters folder. metrics are provided in a nested folder in exporters, which provides challenges when creating nested directory structures as volume mounts in k8s. The metrics config discovery process works the same in the Java code as it did before, but the metrics and exporters folders are now completely separate.
• Add ENV VAR configuration for config folder locations. Currently the path is hardcoded to the examples contained within the source code at resources/exporters/metrics. An environment variable can now be provided for the exporter and metrics directory paths, with the default path pointing to the examples provided within the source code. This is key to allow users to define their own rules without modifying the source code.
• Update prometheus metrics to produce lowercase type values. Prometheus complains about the uppercase metric types (e.g. COUNT, GAUGE). These have been converted to lowercase in the prometheus exporter, as well as in the tests.