Add otlp exporters

[actualwitch]

• init function is now mandatory
• prometheus-client exposition is merged into a unified api inside init
• otlp exporters are added as extra packages
• test isolation! https://github.com/autometrics-dev/autometrics-py/issues/36

[brettimus]

when you have a chance could you document how to run the otlp example somewhere?

[actualwitch]

I resolved the typing issues by using pydantic for runtime type checking, and added some examples for exporters. also, bumped our fastapi example