RFE: prom-label-proxy: enable multiple label values

1. Proposed title of this feature request
  prom-label-proxy: enable multiple label values

2. What is the nature and description of the request?
  - https://github.com/prometheus-community/prom-label-proxy/pull/115
  - https://github.com/prometheus-community/prom-label-proxy/issues/27

3. Why does the customer need this? (List the business requirements here)
  - "Some tenants are using multiple namespaces and they want to use a single grafana instance to see dashdoards with metrics of multiple namespaces."
  - "multiple grafana instances and dashboards instead of a single grafana instance and a single dashboard" 
  - Grafana community operator
  
4. List any affected packages or components.
  prom-label-proxy