Add time units to storage.tsdb.retention.size flag (#6365)

* Add time units to storage.tsdb.retention.size flag

In an effort to reduce confusion with the `m` option of the
`ParseDuration()` function, this commit adds the available time units to
the `storage.tsdb.retention.time` flag to help showcase that there is no
option for months (which could be assumed to be `m`).

If someone were looking to set the retention to six months, they may
mistakenly do so with `6m`, which would reduce their retention to six
minutes.

Signed-off-by: Brooks Swinnerton <bswinnerton@gmail.com>

cmd/prometheus/main.go

@@ -195,7 +195,7 @@ func main() {
 	a.Flag("storage.tsdb.retention", "[DEPRECATED] How long to retain samples in storage. This flag has been deprecated, use \"storage.tsdb.retention.time\" instead.").
 		SetValue(&oldFlagRetentionDuration)
 
-	a.Flag("storage.tsdb.retention.time", "How long to retain samples in storage. When this flag is set it overrides \"storage.tsdb.retention\". If neither this flag nor \"storage.tsdb.retention\" nor \"storage.tsdb.retention.size\" is set, the retention time defaults to "+defaultRetentionString+".").
+	a.Flag("storage.tsdb.retention.time", "How long to retain samples in storage. When this flag is set it overrides \"storage.tsdb.retention\". If neither this flag nor \"storage.tsdb.retention\" nor \"storage.tsdb.retention.size\" is set, the retention time defaults to "+defaultRetentionString+". Units Supported: y, w, d, h, m, s, ms.").
 		SetValue(&newFlagRetentionDuration)
 
 	a.Flag("storage.tsdb.retention.size", "[EXPERIMENTAL] Maximum number of bytes that can be stored for blocks. Units supported: KB, MB, GB, TB, PB. This flag is experimental and can be changed in future releases.").