Add dedicated endpoints for each collector

[MaJaHa95]

Fixes #97

Consistent with my feature request, I added endpoints for each collector:

• /metrics/client
• /metrics/controller
• /metrics/device
• /metrics/port

While maintaining backwards compatibility with /metrics as before.

I appreciate your review--I'm not very familiar with the Prometheus library, and there might be nuances or a better way to achieve this. I've deployed it in my own environment and it seems to work, but I'm certainly open to better alternatives.