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This makes it easier for downstream clients of the
component to add new product types without needing
to update the component source.

Also, adds a client method to return the product, so
that different MetricsService clients can provide
different values.

Also cleans up some unnecessary metrics:: namespace
prefixes.

BUG=415581

Review URL: https://codereview.chromium.org/573403002

Cr-Commit-Position: refs/heads/master@{#297254}