The drill-down user flow for results.html involves selecting sample diagnostics.
Currently, merged histograms contain sample diagnostics that do not indicate anything about the samples' origins. A merged histogram might contain samples from many different stories, for example, and the user might not know which stories to investigate until after displaying sample diagnostics, or even whether they want to drill down by story as opposed to by OS version, for example.
In addition to the sample Breakdowns and any other sample diagnostics produced by the metric, the sample diagnostics table should contain the metadata from the raw histograms that originally contained the samples.

After the new format's serializer is launched, metadata can be added to every sample by metricMapFunction without significantly increasing the size of the json.
https://bugs.chromium.org/p/chromium/issues/detail?id=918208