Support for Persistence Flags for Helm Charts as present in prometheus pushgateway

[himeshkothari]

Want to be able to persist the metrics if prometheus aggregation gateway pod is restarted.

As listed in the prometheus pushgateway helm chart, we can use extraFlags but those are not supported in prometheus-aggregation-gateway

  - args:
    - --persistence.file=/data/pushgateway.data
    - --persistence.interval=5m