The Telemetry object is constructed in ApplicationImp's member initializer
list where nodeIdentity_ is not yet available, resulting in an empty
service.instance.id resource attribute. Add setServiceInstanceId() virtual
method that Application::setup() calls after nodeIdentity_ is known but
before telemetry_->start() creates the OTel resource.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>