feat(otel): add OpenTelemetry instrumentation support

[erikhoward]

Summary

• Add optional OpenTelemetry instrumentation for LLM requests via new contrib/otel package
• Create ContextualTelemetryHook interface for context-aware telemetry (backward compatible)
• Support GenAI semantic conventions for PetalTrace and standard observability tools

Changes

Core (core/)

• Add ContextualTelemetryHook interface extending TelemetryHook with context support
• Update GetResponse(), Stream(), and wrapStreamWithTelemetry() to use contextual hooks when available

New Package (contrib/otel/)

• Hook implementation that creates OTel spans for LLM requests
• GenAI semantic convention attributes (gen_ai.system, gen_ai.request.model, gen_ai.usage.*)
• Configurable options: custom tracer provider, span name function, additional attributes

Usage

import irisotel "github.com/petal-labs/iris/contrib/otel"

hook := irisotel.New()
client := core.NewClient(provider, core.WithTelemetry(hook))
// All LLM calls now emit OTel spans

Test plan

• All existing core tests pass (backward compatibility)
• New contextual telemetry hook tests pass
• OTel hook tests verify span creation, attributes, error handling, context propagation

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