Describe EC2 availability zones at most once per SD load
Describe EC2 availability zones at most once per SD load

Closes #9142.

Signed-off-by: George Brighton <9fd8de5fc2a7c2c0d469b2fff1afde4e5def37ba@gebn.co.uk>

keegancsmith/prometheus,keegancsmith/prometheus,keegancsmith/prometheus,prometheus/prometheus,alileza/prometheus,alileza/prometheus,keegancsmith/prometheus,prometheus/prometheus,prometheus/prometheus,alileza/prometheus,alileza/prometheus,keegancsmith/prometheus,prometheus/prometheus,alileza/prometheus,prometheus/prometheus