minor refactor of backend metadata and metric rendering

[paulbellamy]

fixes #835

[tomwilkie]

I'm not sure I like the reuse of MetadataRow here. I think it would be better to make an interface like this:

type MetadataRowRenderer interface {
  Render(report.Node) []MetadataRow
}

type Latest struct {
  ID string
}

func (l Latest) Render(report.Node) []MetadataRow {
...
}

type Metric struct {
 ID string
}

func (m Metric) Render(report.Node) []MetadataRow {
...
}
...

You could then see these renderers being included in the report such that plugins can add stuff to the details panel. Make sense?

Also, this would allow you to add a field to Latest called Length, and solve #930 in a nicer way.

[tomwilkie]

Could call MetadataRowRenderer "RowTemplate"

[paulbellamy]

Applied tom's feedback verbatim, so merging.