Function is_admin 

pub fn is_admin(metadata: &MetadataMap) -> bool

Extract the authenticated caller’s admin status from gRPC metadata.

A missing x-auth-is-admin header means no AuthService is configured at all (e.g. --disable-auth) — AuthService::call rejects the request before it reaches the inner service when a provider is configured but validation fails, so the header is otherwise always present. Treat this case as trusted admin, matching the existing --disable-auth convention in analytics-web-srv/src/web_server.rs. An unparseable value fails closed to non-admin.