* Decouple MetricsMiddleware from downstream handlers
Remove ls-real-model-name optimization. Within proxyOAIHandler the
request body's bytes are required for various rewriting features
anyways. This negated any benefits from trying not to parse it twice.
Fix#198
- use llama-server's `timings` info if available in response body
- send "-1" for token/sec when not able to accurately calculate
performance
- optimize streaming body search for metrics information
