[Prism] Fix a distribution metric problem when count is zero

[shunping]

Also clarify the situation in the proto spec.

[shunping]

R: @lostluck

[gemini-code-assist]

Summary of Changes

Hello @shunping, 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 resolves an edge case in distribution metric processing where the count of reported elements is zero. It ensures that runners correctly interpret and aggregate these metrics by ignoring potentially misleading sum, min, and max values when no elements have been recorded. Additionally, the change clarifies this behavior within the metric's protocol buffer definition, improving specification robustness.

Highlights

• Metric Handling for Zero Count: Implemented a fix to correctly handle distribution metrics when the count of elements is zero, preventing potential issues with sum, min, and max values during aggregation.
• Protocol Specification Clarification: Updated the protocol buffer specification (metrics.proto) with comments to explicitly state that sum, min, and max fields should be ignored by runners when a distribution metric's count is zero.

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[lostluck]

"may" in specs usually implies the conditional, so we can avoid the "or may not". I think "omit" here makes it clearer. Same for the paired paragraph below.

Suggested change

	// Note that when count is zero, the SDK may or may not send sum, min, and
	// Note that when count is zero, the SDK may omit sum, min, and