Add OpenTelemetry integration for tracing and metrics

[NP-compete]

Summary

Add OpenTelemetry (OTel) integration for distributed tracing and metrics collection.

Current State

• Basic metrics endpoint exists at `/metrics`
• No distributed tracing
• No standardized observability

Expected Outcome

Tracing

Add trace spans for:

• HTTP requests (middleware)
• Tool invocations
• Resource reads
• Prompt generation

```go
import "go.opentelemetry.io/otel"

func TracingMiddleware(next http.Handler) http.Handler {
return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
ctx, span := otel.Tracer("gomcp").Start(r.Context(), r.URL.Path)
defer span.End()

    span.SetAttributes(
        attribute.String("http.method", r.Method),
        attribute.String("http.url", r.URL.String()),
    )
    
    next.ServeHTTP(w, r.WithContext(ctx))
})

}
```

Metrics

Export metrics for:

• Request count by endpoint
• Request latency histogram
• Tool invocation count
• Error count by type

Configuration

```go
type Config struct {
// ... existing
OTelEnabled bool `mapstructure:"OTEL_ENABLED"`
OTelEndpoint string `mapstructure:"OTEL_EXPORTER_OTLP_ENDPOINT"`
OTelServiceName string `mapstructure:"OTEL_SERVICE_NAME"`
}
```

Dependencies

```
go.opentelemetry.io/otel
go.opentelemetry.io/otel/exporters/otlp/otlptrace
go.opentelemetry.io/otel/sdk/trace
go.opentelemetry.io/contrib/instrumentation/net/http/otelhttp
```

Acceptance Criteria

• OTel SDK integrated
• Tracing middleware added
• Tool invocations traced
• Metrics exported in Prometheus format
• Configurable via environment variables
• Documentation for connecting to Jaeger/Grafana