Support otlp_traces_endpoint for trtllm

### 🚀 Feature Description and Motivation

v1.2.0 and above.

```
# disagg_config.yaml (proxy)
otlp_config:
otlp_traces_endpoint: "http://jaeger:4317"
```

the disagg proxy itself does NOT create spans.
```
Client (traceparent) → Proxy (no span) → CTX Worker (llm_request span) -> → GEN Worker (llm_request span)
```

The two worker spans share the same traceparent from the client, so they appear as siblings under the same trace in Jaeger/Zipkin, but there's no parent span from the proxy showing the orchestration (ctx→gen sequencing, routing decisions, etc.).

some limitations
- No proxy-level spans — routing decisions, ctx/gen orchestration timing not traced
- Worker spans are created at request completion, not request start — they're retroactive spans with corrected timestamps
- No automatic span propagation between proxy→worker HTTP calls — the OpenAIHttpClient doesn't inject traceparent headers into forwarded requests (it passes through the original client's headers if present)
- return_perf_metrics: true is required on workers for spans to have meaningful timing data

### Use Case

observability

### Proposed Solution

_No response_