Create a span

Create a request-log span via the logging API. This is the standard create endpoint; /api/request-logs/create/ remains supported as a legacy alias. For LLM request logs, send prompt_messages, completion_message, token counts, timing, metadata, tools, and trace fields directly in the body. generation_time is accepted and normalized to latency; ttft is accepted and normalized to time_to_first_token. The stored environment is derived from the API key environment, so use a key for the target environment rather than relying on a body override. Metadata keys beginning with _ are reserved for platform use and are omitted from customer-facing span and trace responses, so they do not round-trip through read APIs.