Configure EvaluationPeriods on queue alarms

[AnthonySteele]

There can be transient spikes in the "AgeOfOldestMessage" metrics, due to a momentary fault with an upstream dependency. For many queues, this should not cause an alarm.

if they can set the "EvaluationPeriods" to 2, then

• this problem goes away but
• it will take longer to get a genuine alarm (2 periods is 10 minutes)

Should the EvaluationPeriods be increased for all queues or for 1 queue at a time? For both queue alarm types or individually?

[AnthonySteele]

We might be able to use percentile statistics e.g. the p95 soon.

[tomhaigh]

You can already do this for the newer alarm types, so suggest we leave this and move sqs over as well.