Function meter_key 

pub fn meter_key(cache_ns: &str) -> &str

The per-tenant METERING key for a PC-ISO namespace (lane/cache-metering): the tenant half of scope_namespace — keyring deployments aggregate one row per tenant across all its end-user salts; no-keyring namespaces (the raw salt) pass through unchanged (“” = the default single-tenant namespace). Unforgeable for the same reason scope_namespace is: NS_SEP is excluded from tenant ids, so a salt can never move its tokens into another tenant’s row.