make metrics builder more tolerant of missing data
make metrics builder more tolerant of missing data

 * If an exception is thrown early in the request lifecycle (e.g. the
   `url` query parameter is missing in the request), the metrics
   objects and data may not yet be available.  Avoid crashing while
   building the metrics by testing for existence first.  Those that
   have been built will be included, the others will be skipped.

TomBaxter/modular-file-renderer,AddisonSchiller/modular-file-renderer,Johnetordoff/modular-file-renderer,CenterForOpenScience/modular-file-renderer,AddisonSchiller/modular-file-renderer,AddisonSchiller/modular-file-renderer,Johnetordoff/modular-file-renderer,Johnetordoff/modular-file-renderer,AddisonSchiller/modular-file-renderer,felliott/modular-file-renderer,rdhyee/modular-file-renderer,felliott/modular-file-renderer,Johnetordoff/modular-file-renderer,CenterForOpenScience/modular-file-renderer,TomBaxter/modular-file-renderer,felliott/modular-file-renderer,TomBaxter/modular-file-renderer,CenterForOpenScience/modular-file-renderer,rdhyee/modular-file-renderer,TomBaxter/modular-file-renderer,rdhyee/modular-file-renderer,rdhyee/modular-file-renderer,felliott/modular-file-renderer,CenterForOpenScience/modular-file-renderer