tracing,execinfrapb: include child metadata in trace agg metas

[dt]

This captures and includes in aggregated trace info the "child span metadata", i.e. the occurrence count count and sum duration of each uniquely named child span. These are included in the "per-component" and "cluster wide" trace dumps utilized by various jobs' advance debugging tools.

Release note: none.
Epic: none.

[blathers-crl]

It looks like your PR touches production code but doesn't add or edit any test code. Did you consider adding tests to your PR?

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[cockroach-teamcity]

This change is