chore: Update FiraSans to v3.1

FiraSans v3.1 is packaged in connect-fonts-firasans@0.0.3

fixes #1199
Merge pull request #1222 from mozilla/issue-1199-firasans-update

chore: Update FiraSans to v3.1
feature: Prepare metrics for heka.

* Send metrics to stderr after flattening/formatting.