Add more Content API metrics to Cloud Watch

Adds the timeout metric and timing metric to relevant servers.

I've also created a new metric which is just 'any error response' from Content API, to help us figure out which servers are experiencing a high number of errors (which I think will explain the issues we've seen with the circuit breaker).

Once we've got that info, we can do a bit of investigation to find out why, and see if it's either

a) Something we need to fix on those servers 

or 

b) An issue with the threshold we've set for the circuit breaker.