Improve Vert.x Redis stable metric test

[trask]

Summary

• use the shared DB duration metric helper in the Vert.x Redis stable semconv test
• assert the stable metric unit, description, histogram points, and expected attribute keys instead of checking only the metric name

[Copilot]

Pull request overview

This PR strengthens the Vert.x Redis client stable semantic conventions metrics test by switching from a minimal “metric name exists” check to the shared DB client duration metric assertion helper, improving verification of metric metadata and datapoint attribute keys.

Changes:

• Replace the ad-hoc stable-metric presence check with DbClientMetricsTestUtil.assertDurationMetric(...).
• Expand the assertion to validate the stable metric’s unit, description, histogram shape, and expected attribute keys.