Allow specifying additionalLabels sourced from pod labels

**Is your feature request related to a problem? Please describe.**
We would love to expose some pod metadata in the metrics reported by Retina pods, but we are limited to working with the (IP, port, direction, pod_name) set. Can we make it possible to specify any pod label name in `additionalLabels` to instruct Retina to populate that label as the metric label?

**Describe the solution you'd like**
For example, if my pod has a label `importantData: 123` that I want to report alongside the pod name, I would add a `customLabel_importantData` entry under the `additionalLabels` key in the `MetricsConfigurationCRD` and will in return start receiving Prometheus metrics annotated with `{importantData="123"}` labels.

**Describe alternatives you've considered**
N/A

**Additional context**
Sometimes a pod name does not contain all information needed to attribute a time series to a source.