docs: Add changelog entry for QueryThrottler observability metrics

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Documents PR #18740, which adds four new observability metrics to VTTablet's QueryThrottler. The metrics track total requests evaluated, throttled requests, total latency, and evaluation latency. Each metric includes labels for Strategy, Workload, and Priority, with the throttled metric providing additional context about which metric triggered throttling and whether it was in dry-run mode.

Trigger Events

• vitessio/vitess PR #18740: Add metrics to track requests throttled in QueryThrottler

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The CI check failed because the PR is missing required labels. All PRs in the vitessio/vitess repository must have:

1. A Type: label
2. A Component: label

For this changelog documentation PR, you'll need to add:

• Type: Documentation
• Component: VTTablet (recommended, since QueryThrottler is a VTTablet feature)

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