Add KV Cache Manager to track block metrics

[maschad]

Building off #410 we would like to track:

• Accelerator (e.g. GPU) total KV cache block allocation for the node's model(s);

• Accelerator average number of used blocks over the last time period;

Given nvlm doesn't have direct access for this, we have to tracks KV cache block usage over time on our own, this could be stored as a prom metric or even in our own db.