Allow the Prometheus port to be configuration

[thaiphv]

Is your feature request related to a problem? Please describe.
Currently the Prometheus port is hardcoded to 9100 which has already allocated to the node_exporter in https://github.com/prometheus/prometheus/wiki/Default-port-allocations

Describe the solution you'd like
The port should be configurable so that if someone also deploys a node_exporter on the same host, the server will still work at another port.

Describe alternatives you've considered
N/A

Additional context
N/A