Add metrics to the Python OpenAI instrumentation

[drewby]

Description

This PR implements the GenAI semantic conventions for the two client metrics so they are collected along with spans when instrumenting a Python application.

Basic implementation of two client metrics defined in the GenAI semantic conventions:

gen_ai.client.token.usage - Documentation
gen_ai.client.operation.duration - Documentation

There is an example added to show end users who to configure the explicit bucket boundaries as defined in the semantic convention spec.

Fixes #3177

Type of change

Please delete options that are not relevant.

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

How Has This Been Tested?

Please describe the tests that you ran to verify your changes. Provide instructions so we can reproduce. Please also list any relevant details for your test configuration

• test_chat_completion_metrics - tests that metrics are correctly emitted when calling OpenAI synchronously.
• test_async_chat_completion_metrics - tests that metrics are correctly emitted when calling OpenAI asynchronously.

Does This PR Require a Core Repo Change?

• Yes. - Link to PR:
• No.

Checklist:

See contributing.md for styleguide, changelog guidelines, and more.

• Followed the style guidelines of this project
• Changelogs have been updated
• Unit tests have been added
• Documentation has been updated

[xrmx]

A couple of nits but LGTM. Not sure that setting the views for having proper buckets it's useful but no big deal.

[lmolkova]

Looks great, just a few comments on recording errors