Add requests to an error budget dashboard

When we're done with &269 we should include information about web requests in a dashboard. We currently have this dashboard showing the information for sidekiq.

To add the error rates per feature category, we'll first need to add the feature_category label to the http_requests_total, which should also receive the status label so we can use that counter rather than the histogram for error rates.

When feature_category is exposed on the right metrics, we can add it to the significantLabels. It will be included in the generation of the recording rules.

This will make it visible on the web dashboard in the component (puma) detail.

We can then duplicate the Dashboard from Sidekiq using the newly generated recording rules in a first iteration. We can't combine it in an overall error-budgetting dashboard, because we have yet to figure out the weighting of the web-error rates vs the sidekiq ones. This new dashboard could get the ball rolling.