We currently allow groups of histograms to be created using <histogram-suffixes> declarations, but the syntax for these is fairly inflexible, and doesn't match the kind of code people typically write to create these families.

We should probably have a syntax that declares a pattern of histograms, with seperate declarations of variants of families of histograms, e.g.:

<name-piece name="MyFeatureColor">
  <variant name="Red">
  <variant name="Green">
  <variant name="Blue">
</name-piece>

<name-piece name="MyFeatureSize">
  <variant name="Small">
  <variant name="Medium">
  <variant name="Large">
</name-piece>

<histogram-pattern name="MyFeature.{color}.{size}.Result" enum="BooleanSuccess">
  <owner>me@google.com</owner>
  <summary>The result of MyFeature operation for a color and size.</summary>
  <token key="color" variant="MyFeatureColor">
  <token key="size" variant="MyFeatureSize">
</histogram-pattern>

<histogram-pattern name="MyFeature.{color}.{size}.Time" units="ms">
  <owner>me@google.com</owner>
  <summary>How long MyFeature operation took for a color and size.</summary>
  <token key="color" variant="MyFeatureColor">
  <token key="size" variant="MyFeatureSize">
</histogram-pattern>