Token usage

AsyncOpenAICompatibleClient.usage and AsyncOpenAIEmbedder.usage return a TokenUsage snapshot counted across every response the client has received, including responses whose body was then rejected:

async with pipeline:
    await pipeline.run(query="all:galaxy", total_limit=50)
usage = llm.usage
print(f"{usage.requests} requests, {usage.prompt_tokens} prompt and {usage.completion_tokens} completion tokens")

TokenUsage has requests, prompt_tokens, completion_tokens, and total_tokens. A response without usage data counts as a request with zero tokens. Other AsyncLLMClient and AsyncEmbedder implementations return None unless they override the usage property.