chart/network-observer: Control Prometheus configuration

[c-kruse]

Is your feature request related to a problem? Please describe.
Advanced users may want control over prometheus configuration for a number of reasons

• Setting storage parameters like retention, or backing storage
• Excluding certain metrics from collection (such as internal metrics or traffic metrics when not permissible.)
• Configuring a remote write

Describe the solution you'd like
Expose values in the helm chart to allow users to overwrite:

• The prometheus container arguments ( e.g. for setting --storage.tsdb.retention.size )
• The prometheus data storage volume details (e.g. inserting a volume backed by persistent storage)
• The prometheus configuration file contents

Describe alternatives you've considered
A clear and concise description of any alternative solutions or features you've considered.

Additional context
Add any other context or screenshots about the feature request here.