Updates to OpenAI Spans and Metrics conventions

[drewby]

Changes

Here are some proposed changes based on the conversation in your PR. I know we haven't agreed to move LLM history to events, so I'm just suggesting here what it might look like if it were events.

• Adds OpenAI Metrics from @lmolkova fork.
  • I added llm prefix to her metric names.
• Updates to OpenAI Spans per our conversation on your PR.
  • I repeated the llm attributes in addition to the openai. This is similar to other conventions where "server.address" gets repeated for clarity.
  • I updated names and added new attributes from the recent API changes (functions are not tools, addition of seed and system_fingerprint, etc
  • Moved Messages, Tools, Choices to Span Events and made them optional with a SHOULD be configured by user of instrumentation libraries.