Add support for async functions

[flenter]

Right now the decorator assumes that the function it is wrapping when it returns it is also finished. This is not the case with async functions.

async def async_function():
    """This is an async function."""
    await asyncio.sleep(0.1)
    return True

When calling this function it will return quickly but when you use await it suddenly takes ~0.1s longer.

[brettimus]

Adding a note here: Decorating fastapi path operation functions with @autometrics breaks the API

So, this throws an error:

@app.get("/async-test")
@autometrics
async def async_test_route():
    message = await my_async_function()
    return {"Hello": message}

But this works:

@app.get("/async-test")
async def async_test_route():
    message = await my_async_function()
    return {"Hello": message}