feat(telemetry): Weaver live-check, registry catch-up, reference collector

[krisztianfekete]

Last part of #2907.

• Weaver: bumped to v0.27.0 and the GenAI conventions pin to 06ec68e7. The registry version is now 0.2.0.
• Registry:
  • Defines a2a.request as a span (kagent.a2a.request). Drops the SERVER span refinements, since nothing annotates SERVER spans.
  • Imports the upstream GenAI, HTTP and RPC spans and metrics the runtimes emit.
  • Adds a kagent.resource group for live-check.
• Metrics: Go and Python record gen_ai.invoke_agent.duration on the invoke_agent spans kagent opens.
• e2e: the suite now captures metrics as well as traces. It replays both to weaver registry live-check (report-only), configured in telemetry/conformance/live-check.toml.
• Reference collector: examples/observability/collector-values.yaml, installed with make otel-collector-kind.

Breaking changes

• Runtime resource attributes are renamed from gen_ai.agent.name and gen_ai.agent.id to gen_ai.main_agent.name and gen_ai.main_agent.id, following the upstream gen_ai.main_agent entity. Spans keep gen_ai.agent.*.
• Go request spans no longer carry kagent.runtime; it is on the resource only, as in Python.