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! Pytest should clear collected metrics after each test runs #36

[brettimus]

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

[brettimus]

should this (and other configuration lines) use safe accessors + have a default? i don't think it's guaranteed to be defined

alternatively, we could merge it with a default dict, to avoid fallback madness

[actualwitch]

most of these are marked as Optional so it's ok if they are just forwarded as None's. i'm pretty sure they all also have defaults/not requred

[actualwitch]

ah i see what's happening - i had to add the whole get_exporter_settings function to make it type typeddicts correctly but this now requires all fields to be filled. i'll look into refactoring this in a more sane way

[brettimus]

right on, your latest changes fixed it! as far as i can tell, the option names are the same between our library and the otel one for the otlp exporter, so it might be good to drop a link to their documentation somewhere for available options? if any thing just for our future selves

[brettimus]

so i needed to fill in all fields like this to get things working (related to my comment about providing defaults for the exporter config):

    exporter={
        "type": "otlp-proto-http",
        "endpoint": "http://localhost:4318/v1/metrics",
        "insecure": True,
        "headers": None,
        "credentials": None,
        "push_interval": 1000,
        "timeout": 1000,
        "preferred_temporality": None,
    },

[actualwitch]

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