feat(observability): add LangWatch exporter for AI tracing

[sergioestebance]

Summary

• Adds @mastra/langwatch package — a first-class LangWatch exporter for Mastra observability
• Replaces the manual OtelExporter configuration with a dedicated zero-config exporter
• Extends OtelExporter (same pattern as @mastra/arize) with pre-configured OTLP endpoint and Bearer token auth

Changes

New package: observability/langwatch/

• LangwatchExporter class extending OtelExporter (~60 lines)
• Pre-configured endpoint: https://app.langwatch.ai/api/otel/v1/traces
• Protocol: OTLP/HTTP (protobuf)
• Auth: Bearer token via Authorization header
• Zero-config via LANGWATCH_API_KEY env var
• Custom endpoint via LANGWATCH_ENDPOINT for self-hosted instances
• 5 unit tests covering all configuration paths

Documentation

• Docs page: docs/observability/tracing/exporters/langwatch.mdx
• Reference page: reference/observability/tracing/exporters/langwatch.mdx
• Added to docs and reference sidebars
• Added to tracing overview external exporters list
• Migration guide from manual OtelExporter setup

Changeset

• @mastra/langwatch: major (initial release)

Usage

import { LangwatchExporter } from "@mastra/langwatch";

// Zero-config (reads LANGWATCH_API_KEY env var)
new LangwatchExporter()

// Explicit config
new LangwatchExporter({ apiKey: "lw_xxx" })

// Self-hosted
new LangwatchExporter({ endpoint: "https://your-instance.example.com/api/otel/v1/traces" })

Test plan

• pnpm test passes in observability/langwatch/ (5/5 tests)
• pnpm build --filter=@mastra/langwatch succeeds
• Docs build without errors
• Manual test: configure with real LANGWATCH_API_KEY and verify traces appear in LangWatch dashboard

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