Add test to verify that ConsumeMetrics and ConsumeTraces clear response members

[ddelnano]

Summary: Add test to verify that ConsumeMetrics and ConsumeTraces clear response members

This adds test coverage for the bug fix in #1910. This is a follow up to the conversation here

Relevant Issues: N/A

Type of change: /kind bug

Test Plan: Verified that unit test fails if #1910 is reverted

$ git show HEAD
commit 4ab4a9ce58bd394e4cd8287b44767e870fedd127 (HEAD -> ddelnano/add-tests-for-otel-sink-bug, ddelnano/ddelnano/add-tests-for-otel-sink-bug)
Author: Dom Del Nano <ddelnano@gmail.com>
Date:   Fri Jul 26 12:17:00 2024 +0000

    Revert "Clear trace response instead of metric response in `OTelExportSinkNode::ConsumeSpans` (#1910)"

    This reverts commit 970a54a7b9deb9469c406d73069c1e310a53ea96.

$ bazel test src/carnot/exec:otel_export_sink_node_test --test_output=all

[ ... ]
[ RUN      ] OTelExportSinkNodeTest.consume_spans_clears_span_responses
src/carnot/exec/otel_export_sink_node_test.cc:1748: Failure
Value of: response->partial_success().rejected_spans() == 0
  Actual: false
Expected: true