add(logging): Add OTel logging for counting chars

[kiranani]

TLDR

We only have added and removed lines included in OTel. This change adds the character removals and additions too

Testing Matrix

	🍏	🪟	🐧
npm run	❓	❓	✅

Linked issues / bugs

[gemini-code-assist]

Summary of Changes

Hello @kiranani, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request improves the granularity of telemetry data collected for code modifications. By adding character-level diff statistics to the existing line-level metrics, the system will gain deeper insights into how code changes are made, whether by the model or the user. This enhancement provides a more comprehensive understanding of code evolution and impact.

Highlights

• Enhanced Telemetry Data: This PR extends OpenTelemetry logging to capture character-level diff statistics, specifically model_added_chars, model_removed_chars, user_added_chars, and user_removed_chars. Previously, only line-level statistics were collected.

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[gemini-code-assist]

Code Review

This pull request successfully adds character count metrics to the OpenTelemetry logging for tool calls that produce a diff. The changes correctly propagate the new diffStat fields through the ToolCallEvent and update the corresponding tests. My review includes one suggestion to improve code maintainability in packages/core/src/telemetry/types.ts by simplifying how the diffStat object is assigned to the event metadata.