Top product description fallback never fires when operator or product name is missing

[StuartFerguson]

In BuildTopProductMetric, the description is built with an interpolated string and then ?? "Unknown product".

Impact:

• The fallback is unreachable
• Null or empty operator/product names can surface as malformed labels

Suggested fix:

• Check OperatorName and ProductName explicitly before formatting, or coalesce each value separately.