Add support for decorating async functions

[brettimus]

This PR adds support for async functions, making the following changes:

• Conditionally return a sync or async wrapped function, depending on whether the decorated function returns true from iscoroutinefunction
• Write an additional overload for the autometrics decorator, using Callable[P, Awaitable[T]] as part of the type signature
• Factor out shared logic from decorator and async_decorator
• Add a test of a simple async function in test_decorator.py (which required adding the pytest-asyncio module to dev dependencies)
• Add async code to the fastapi-example.py example

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Fixes #29

[brettimus]

this logic might not need to live here too — i could try factoring out of deocrator and async_decorator

[flenter]

Nice work! Got a couple of minor points of feedback though

[flenter]

🚀

[brettimus]

i'm actually not quite sure what this accomplishes! now that we have two different decorators (one for sync, one for async) i am a slightly worried about returning the sync decorator here when the func arg is None.

i did try a version of this PR that used one decorator, but to get the value of an async function synchronously, you have to do some tricks with the event loop which seemed like they were against best practices. e.g.,:

import asyncio

async def async_function():
    # Your async code here
    return result

def sync_function():
    loop = asyncio.get_event_loop()
    result = loop.run_until_complete(async_function())
    return result

anyways, we don't seem to document this case (where the decorator is called without being attached to a function), so this might not be a problem

[flenter]

Instead of returning different decorators, would it be possible to use only one and check whether the result is a coroutine?
https://docs.python.org/3/library/inspect.html#inspect.iscoroutine