Clear trace response instead of metric response in `OTelExportSinkNode::ConsumeSpans`

[benkilimnik]

Summary: Addresses a couple of small-ish bugs

• ConsumeSpans is mistakenly clearing the metrics_response_ (which is used in ConsumeMetrics) instead of the trace_response_.
• There is a duplicate return statement in OTelModule::Init.

Type of change: /kind bug

Test Plan: CI, skaffold-ed standalone pem and ran OTel export scripts