feat: Add Pydantic AI instrumentation via OpenTelemetry

Add OpenTelemetry-based instrumentation for Pydantic AI agents:

- PostHogSpanExporter: Generic OTel exporter that translates spans to PostHog events
- PydanticAISpanExporter: Wrapper that normalizes Pydantic AI message formats and tool attributes
- instrument_pydantic_ai(): One-liner setup function for easy integration

Usage:
    from posthog.ai.pydantic_ai import instrument_pydantic_ai
    instrument_pydantic_ai(posthog_client, distinct_id="user_123")

    agent = Agent("openai:gpt-4")
    result = await agent.run("Hello")  # Automatically traced

Co-Authored-By: Claude <[email protected]>

Author: andrewm4894

posthog/ai/otel/README.md

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+# PostHog OpenTelemetry Integration
+
+This module provides a generic OpenTelemetry `SpanExporter` that translates OTel spans into PostHog AI analytics events.
+
+## Overview
+
+Many AI/LLM frameworks use OpenTelemetry for instrumentation. This exporter allows PostHog to receive telemetry from any OTel-instrumented framework by converting spans to PostHog events.
+
+```
+┌─────────────────┐     ┌──────────────┐     ┌─────────────────────┐     ┌─────────┐
+│ AI Framework    │────>│ OTel Spans   │────>│ PostHogSpanExporter │────>│ PostHog │
+│ (Pydantic AI,   │     │ (native)     │     │ (translates spans)  │     │ Events  │
+│  LlamaIndex...) │     └──────────────┘     └─────────────────────┘     └─────────┘
+└─────────────────┘
+```
+
+## Usage
+
+```python
+from opentelemetry.sdk.trace import TracerProvider
+from opentelemetry.sdk.trace.export import BatchSpanProcessor
+from posthog import Posthog
+from posthog.ai.otel import PostHogSpanExporter
+
+# Create PostHog client
+posthog = Posthog(project_api_key="phc_xxx", host="https://us.i.posthog.com")
+
+# Create exporter and tracer provider
+exporter = PostHogSpanExporter(
+    client=posthog,
+    distinct_id="user_123",
+    privacy_mode=False,  # Set True to exclude message content
+)
+
+provider = TracerProvider()
+provider.add_span_processor(BatchSpanProcessor(exporter))
+
+# Use this provider with your OTel-instrumented framework
+```
+
+## Span to Event Mapping
+
+The exporter classifies spans and maps them to PostHog events:
+
+| Span Type | PostHog Event | Detection |
+|-----------|---------------|-----------|
+| Model request | `$ai_generation` | Span name starts with "chat" or has `gen_ai.request.model` |
+| Tool execution | `$ai_span` | Span name contains "tool" or has `gen_ai.tool.name` |
+| Agent orchestration | (skipped) | Span name contains "agent" |
+
+## GenAI Semantic Conventions
+
+The exporter follows [OpenTelemetry GenAI semantic conventions](https://opentelemetry.io/docs/specs/semconv/gen-ai/):
+
+| OTel Attribute | PostHog Property |
+|----------------|------------------|
+| `gen_ai.request.model` | `$ai_model` |
+| `gen_ai.system` | `$ai_provider` |
+| `gen_ai.usage.input_tokens` | `$ai_input_tokens` |
+| `gen_ai.usage.output_tokens` | `$ai_output_tokens` |
+| `gen_ai.input.messages` | `$ai_input` |
+| `gen_ai.output.messages` | `$ai_output_choices` |
+| `gen_ai.tool.name` | `$ai_span_name` |
+| `gen_ai.tool.call.arguments` | `$ai_tool_arguments` |
+| `gen_ai.tool.call.result` | `$ai_tool_result` |
+
+## Configuration Options
+
+| Parameter | Type | Description |
+|-----------|------|-------------|
+| `client` | `Posthog` | PostHog client instance |
+| `distinct_id` | `str` | User identifier (falls back to trace ID if not set) |
+| `privacy_mode` | `bool` | Exclude message content from events |
+| `properties` | `dict` | Additional properties to include in all events |
+| `groups` | `dict` | PostHog groups for all events |
+| `debug` | `bool` | Enable debug logging |
+
+## Framework-Specific Exporters
+
+For frameworks with non-standard attribute names or message formats, use the framework-specific exporter wrapper:
+
+- **Pydantic AI**: Use `posthog.ai.pydantic_ai.PydanticAISpanExporter` or the simpler `instrument_pydantic_ai()` function
+
+These wrappers normalize framework-specific formats before passing spans to `PostHogSpanExporter`.

posthog/ai/otel/__init__.py

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+"""
+OpenTelemetry integration for PostHog AI observability.
+
+This module provides a SpanExporter that translates OpenTelemetry spans
+(particularly GenAI semantic convention spans) into PostHog AI events.
+"""
+
+from posthog.ai.otel.exporter import PostHogSpanExporter
+
+__all__ = ["PostHogSpanExporter"]