[EPIC] OTel GenAI compliant telemetry contract

[krisztianfekete]

We have a bit of a mess currently with weird tracing/logging logics, inconsistent telemetry emission across frameworks/runtimes and using outdated SDKs layered on top of all this.

This issue tracks the initiative to consolidate all telemetry into a Weaver-backed registry, enforce our rules around attributes, cardinality, make all runtimes/frameworks consistent, and have versioned compliance with OTel GenAI semantic conventions.

Tasks:

• feat(telemetry): add a Weaver registry as the telemetry contract #2906
• feat(telemetry): configure runtimes with SDK-spec OTEL variables #2909
• feat(telemetry): export semconv metrics, drop ADK legacy spans, correlate logs #2929
• Moving Python off OpenLLMetry
• Enforcing conformance via Weaver

Upstream dependencies:

• feat(telemetry)!: default to experimental GenAI semconv, with ADK_TELEMETRY_SCHEMA_VERSION_OPT_IN=otel_semconv_1_36 rollback google/adk-go#1635