Function describe_key 

pub fn describe_key(
    vault: &mut Vault,
    key: &str,
    description: &str,
    example: Option<&str>,
    tags: &[String],
    rotation_interval_days: Option<Option<u32>>,
    expires_at: Option<Option<&str>>,
)

Update a key’s plaintext schema metadata.

rotation_interval_days and expires_at are tri-state patches so a describe that omits them never clobbers sticky rotation policy:

• None — leave the existing value untouched
• Some(None) — clear it
• Some(Some) — set it