Add tracking for high watermark cache usage and make it available in /metrics endpoint.
Use-case: Tracking largest needed cache usage under realistic workload
to better understand memory requirements and be able to adjust
cache size/quantization for model/cache accordingly.
There was a typo-like error, which would print the same number twice if
request is received with n_predict > server-side config.
Before the fix:
```
slot launch_slot_: id 0 | task 0 | n_predict = 4096 exceeds server configuration, setting to 4096
```
After the fix:
```
slot launch_slot_: id 0 | task 0 | n_predict = 8192 exceeds server configuration, setting to 4096
```