metrics: Add pending builds support.

* src/cuirass/metrics.scm (db-pending-builds, db-current-day-timestamp): New
procedures.
(%metrics): Add 'pending-builds.
(db-update-metrics): Add it.
* src/cuirass/templates.scm (global-metrics-content): Add "pending-builds"
argument. Display pending builds in a line chart.
* src/cuirass/http.scm (url-handler): Adapt "global-metrics-content" call
accordingly.