report: render notApplicable metrics with double dash

[brendankenny]

part of #13916

@adamraine convinced me in that issue that we should just use scoreDisplayMode notApplicable to allow metrics to declare themselves not-applicable, but (unlike all other audits) still be displayed in the report. This allows for "optional" metrics like responsiveness to correctly appear in timespan reports, even if there were no interactions in that timespan.

I still think there's a chance we won't want to eliminate the only way audits can dynamically remove themselves from the report, but that chance seems very slim since we've never done this with a metric before in the four years since metric rendering was changed much, and I think we'll still be able to change our minds without really breaking backwards/forwards compatibility with report rendering.

All that said: how should this look? Almost everything was already working so this PR is just adding some placeholder text. It's pretty weak right now, open to ideas :)

• better styling to offset it slightly?
• @paulirish had proposed having custom text there (e.g. "No interactions in timespan" or something for INP), which is possible, since notApplicable audits can still have their displayValue set and could set their own string there. Would that be better? Would need to make sure the string (and translations) fit.

[brendankenny]

LHR for anyone who wants to bikeshed: https://gist.github.com/6be296a3c582d36e850ae1d102270743

[connorjclark]

I'd prefer N/A here, even though we'd need to translate it (we could mention to the translators if they can't keep it short, then just provide --). but don't have any strong opinions on this.

[adamraine]

better styling to offset it slightly?

Could it be gray like other N/A stuff

@paulirish had proposed having custom text there (e.g. "No interactions in timespan" or something for INP), which is possible, since notApplicable audits can still have their displayValue set and could set their own string there. Would that be better? Would need to make sure the string (and translations) fit.

I like the idea of using display value. Just doing "N/A" or "--" doesn't really explain why the metric is absent. Could use "--" or "N/A" as a fallback if display value is falsy?

[connorjclark]

double dash is good for now, we can bikeshed this more post 9.6

[connorjclark]

wtf why didn't the commit take the PR title