Function is_valid_entity_type 

fn is_valid_entity_type(ty: &str) -> bool

Returns true if ty is a syntactically valid entity type, e.g. service, host, k8s.pod, service.instance. An entity type is one or more dot-separated segments, each a non-empty [a-z0-9_]+ token. The dotted form carries the two-entity-layer convention (service vs service.instance).