feat: add AutoTracingPlugin for OpenTelemetry auto-instrumentation

Monkey-patches in-scope Python functions to emit an OpenTelemetry span
per call, capturing args/return/exception as span attributes. Scope is
auto-discovered from the agent tree. No-op when the tracer is not
recording.

Change-Id: I103678b2189a75dcd74af51deb75eb4346c20551