feat(metrics): set metricsContext expiry to 2 hours
feat(metrics): set metricsContext expiry to 2 hours

#1519

Fixes #1513

r=philbooth
mozilla/fxa-auth-server,eoger/fxa-auth-server,mozilla/fxa-auth-server,mozilla/fxa-auth-server,mozilla/fxa-auth-server,eoger/fxa-auth-server,eoger/fxa-auth-server,eoger/fxa-auth-server