SLO Aware Routing Plugins and Metrics

[BenjaminBraunDev]

This PR contains the (now combined) stages 2/3 and 3/3 for adding in the latency prediction functionality to EPP.

New features:

1. New -enable-latency-predictor flag in EPP arg to inform it that sidecars are present and to register slo routing plugins.
2. Requests now support latency SLOs in their header (x-slo-ttft-ms and x-slo-tpot-ms) and a boolean for whether to use the SLO routing scheduling profile with slo scoring (x-prediction-based-scheduling). If false, use the default profile and just track and train for future requests.
3. Tracks requests and their latencies and sends data back to training sidecar for processing.
4. Scores based on latencies and headroom.
5. Adds (back) the totalRunningRequestsMetric prometheus metric from vLLM, which was removed for being unused in the past, but is now a feature of our latency prediction model.

Fixes #1323

Does this PR introduce a user-facing change?:

Yes:
EPP has a new runtime/deployment argument: `-enable-latency-predictor`

When enabled EPP, will register and support the new SLO routing plugins and sidecars. With this flag, EPP will assume the predictor and training sidecars also exist in its same container.

When latency prediction is enabled for EPP, requests support 3 new headers:
- `x-slo-ttft-ms` is a float for the desired "Time To First Token" SLO
- `x-slo-tpot-ms` is a float for the desired average "Time Per Output Token" SLO
- `x-prediction-based-scheduling` is a boolean for whether to route this request with the default scheduling profile or with the slo-aware scheduling profile

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