fix(crons): Make monitor async friendly

[sentrivana]

Our monitor decorator/context manager is not async friendly. If you wrap an async function in it, monitor will only wrap the creation of the coroutine, rather than its execution, which means it'll exit early. It won't capture the actual duration of the execution of the function, and the cron run will always be reported as success.

To better illustrate, try this out:

import asyncio
import sentry_sdk

sentry_sdk.init(...)

@sentry_sdk.monitor(monitor_slug='my-slug')
async def my_cron():
    print('running...')
    await asyncio.sleep(1)
    raise ValueError

asyncio.run(my_cron())

If you execute this, you'll see multiple fishy things:

• the cron will be green even though it throws an exception
• even if it takes >= 1s to execute, the time reported will be ~0ms
• the "running..." message will be printed after the final checkin (you can see this with debug=True)

To fix this, we need special handling for the async case where we actually execute the coroutine and wait until it's finished before reporting.

Implementation Notes

This is annoying to implement cleanly because of Python 2 compatibility. Python 2 will throw SyntaxErrors when it encounters any async defs. We could use an exec for the async part, but this is ugly.

Instead, I'm implementing the Python2+3-friendly part of the decorator in the main monitor class and outsourcing the problematic __call__ definition to version-specific mixins, where the Python 3-only mixin will never get imported on Python 2.

In any case, we can get rid of this compat hack with SDK 2.0.