Align `/ingest` HTTP API validation with Push API

The `/ingest` HTTP API has a long history and imposes stricter requirements on the `service_name` label (previously referred to as the "app name" in pre-v1 versions). Specifically, in the Push API, `service_name` supports UTF-8, whereas in the `/ingest` HTTP API, the allowed character set is more restricted (e.g., it does not permit / in the service name).