[prism] Metrics from runner expanded composites aren't mapped back to user terms.

[lostluck]

For example: SDFs and Combines expand out to a few sub transforms, but their metrics aren't visible in the prism UI.

Note how the GenerateRays SDF has an output pcollection, but reports 0 elements, and similarly, TraceFn which consumes it, reports 0 elements input, but emitted several itself.

This appears to primarily affect output PCollection metrics, and the transform's Profiling metrics .

Largely this means summing in the profile metrics from the exploded stages, but in some ways ignoring/discarding metrics from the PCollections of those stages.

This only affects a merged view however. In a view of the fused + generated transforms, they can be properly ascribed to their source components.