Updating storage.md for retention.size (#7942)
Updating storage.md for retention.size (#7942)

Signed-off-by: venkatbvc <172e85c0257c327679d4bc432a91dc28a604d557@gmail.com>
alileza/prometheus,GoogleCloudPlatform/prometheus,prometheus/prometheus,alileza/prometheus,alileza/prometheus,GoogleCloudPlatform/prometheus,prometheus/prometheus,prometheus/prometheus,prometheus/prometheus,alileza/prometheus,GoogleCloudPlatform/prometheus,GoogleCloudPlatform/prometheus,alileza/prometheus,keegancsmith/prometheus,keegancsmith/prometheus,GoogleCloudPlatform/prometheus,keegancsmith/prometheus,keegancsmith/prometheus,keegancsmith/prometheus,prometheus/prometheus