[NEW] Add prometheus port config

[thaiphv]

[NEW] Allow the Prometheus port to be configurable

Closes #11114

This also picks a default port number that doesn't conflict with existing allocations from https://github.com/prometheus/prometheus/wiki/Default-port-allocations. Per the recommendation, I use the 9451 port, which is available after the one allocated to Habitat Exporter

[CLAassistant]

Thank you for your submission, we really appreciate it. Like many open source projects, we ask that you all sign our Contributor License Agreement before we can accept your contribution.
3 out of 4 committers have signed the CLA.

✅ stuartpb
✅ engelgabriel
✅ thaiphv
❌ brylie
_{You have signed the CLA already but the status is still pending? Let us recheck it.}

[geekgonecrazy]

My only problem is this now becomes a breaking change. Because it was released with 9100 already, so if we change the default then when people upgrade it's broken

[thaiphv]

My only problem is this now becomes a breaking change. Because it was released with 9100 already, so if we change the default then when people upgrade it's broken

I'm OK with not setting a default value of 9100 as we haven't actually reserved any port for the service.

[thaiphv]

I've changed the default value of Prometheus_Port to 9100 so that it doesn't introduce a backward incompatible change.

[thaiphv]

Can someone please take a look at this change?