Add OpenTelemetry support for traces (and metrics)

### Summary

The [Edge Proxy](https://github.com/Flagsmith/edge-proxy) currently has no OpenTelemetry integration. For self-hosted deployments that run Edge Proxy as a critical path component (flag evaluation in front of apps), this makes it hard to observe latency, errors, and upstream behaviour in a standard way.

The main [Flagsmith](https://github.com/Flagsmith/flagsmith) API already supports OTel export; Edge Proxy would benefit from similar support so operators can monitor both tiers consistently.

### Use case

Edge Proxy sits between application SDKs and the Flagsmith core API. Operators typically want visibility into:

- **HTTP request handling** — latency, status codes, route-level spans (`/api/v1/flags/`, `/api/v1/identities/`, health endpoints)
- **Upstream polling** — spans/timing for refreshes against the Flagsmith API (`API_URL`, poll frequency/timeouts)
- **Cache behaviour** — cache hit/miss or refresh events (especially with `ENDPOINT_CACHES` configured)
- **Errors** — failed upstream requests, timeouts, proxy errors

Today this is limited to structured logs (`structlog` / `LOGGING` config). That helps, but does not integrate with OTLP-based observability stacks.

### Expected behaviour

Opt-in OTel export via standard environment variables, aligned with the main Flagsmith OTel approach:

```
OTEL_EXPORTER_OTLP_ENDPOINT
OTEL_EXPORTER_OTLP_PROTOCOL # support grpc and http/protobuf
OTEL_SERVICE_NAME # default: e.g. flagsmith-edge-proxy
OTEL_RESOURCE_ATTRIBUTES
OTEL_TRACES_EXPORTER # otlp | none
OTEL_SDK_DISABLED
```

When disabled (endpoint unset or `OTEL_SDK_DISABLED=true`), there should be no runtime overhead — same principle as the main Flagsmith docs.

### Suggested scope

**Minimum (high value):**
- FastAPI/uvicorn auto-instrumentation for incoming HTTP requests
- Manual spans or instrumentation for upstream poll cycles
- OTLP export with **gRPC and HTTP** support

**Nice to have:**
- Metrics (request rate, latency histograms, cache hit ratio, upstream poll duration)
- Propagation of W3C `traceparent` through proxy requests where applicable
- Docs update in Edge Proxy configuration reference

### Current state

- Stack: FastAPI, uvicorn, structlog (see `pyproject.toml`)
- No OpenTelemetry dependencies or instrumentation in the repo today
- Health endpoints exist (`/proxy/health/liveness`, `/proxy/health/readiness`) and should remain excludable from tracing if desired

### Environment

- Edge Proxy version: `2.23.0` (Docker image)
- Deployment: self-hosted, multiple environments