docs(google-adk-go): add native metadata tracing guide - #1075

Summary

Add a metadata-only Google ADK for Go guide using upstream native OpenTelemetry spans and the standard Collector. Keep this PR draft: content-card normalization and a persisted product trace-view walkthrough remain separate acceptance gates. No new SDK, callback span engine, merge, or release is included.

Changes

• Cover google.golang.org/adk v1.9.0 / Go 1.25.0 and google.golang.org/adk/v2 v2.6.0 / Go 1.26.6 plus latest v2.7.0 / Go 1.26.10. The customer-facing v2 example uses v2.7.0; compatibility fixtures retain v2.6.0.
• Keep every enclosing application span and native ADK span in the same trace instead of dropping application ancestors; sanitize names, scopes, resources, schema URLs, attributes, events, links, and status messages before export. ADK v2.7.0 additionally emits a native workflow span around the agent.
• Use upstream baggagecopy with an adk.session.id allowlist on each invocation context, mapping it to session.id without replacing an explicit session ID or native conversation ID. This preserves model and tool membership for concurrent sessions.
• Restrict the application-to-Collector hop to loopback, preserve errors and token metadata, fail closed on transform errors, and send only OTLP/HTTP traces to the Future AGI endpoint. Do not add an ADK log exporter or duplicate callback spans.
• Keep executable regression fixtures and a CI job that installs pinned dependencies first, then tests with localhost-only networking and synthetic credentials.

Verification

• Failing-first reproductions of both original P1 findings were recorded in the prior repair evidence; both fixes are now pushed on this PR branch.
• 52/52 native runtime matrix cases passed locally across v1.9.0, v2.6.0 and v2.7.0, including ancestry, concurrent session propagation, explicit ID precedence, workflows, privacy, malformed fail-closed batches, errors, cancellation, streams, and provider ownership.
• All three exact customer examples passed against a localhost synthetic Gemini API and a real Collector/protobuf receiver: 4 spans on v1/v2.6, 5 spans on v2.7 (one native workflow span); each retained both model calls and their session token counts.
• Collector validation, 26 Node snippet/catalog tests, catalog typecheck, docs/Pagefind build and link audit (0 broken nav or content links) passed. New-head GitHub CI is pending.
• Prior isolated production-reader fixtures covered persisted session retrieval with reduced ClickHouse schemas, not a full product API/UI test. No real Google key or Future AGI Cloud credentials were used.

Remaining acceptance gates

• Native v2 content payloads do not render as prompt/response cards in current trace-details functions; this guide intentionally exports metadata only. Content-schema normalization requires its own implementation and verification before opt-in capture can be offered.
• Persisted product UI trace inspection and a safe product screenshot remain unverified. The guide makes no product-view parity claim. Do not mark this draft ready solely on local fixtures.

Rollback removes this additive guide, catalog entry and isolated examples/workflow. There are no product API, database migration, entitlement or OSS/EE boundary changes.

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