add sync and async test guide at contributing.md

[zhihali]

What problem do you want to solve?

at contrib repo, usually the current test is sync tests, but we sometimes need to run async test. If we just add a async test in our current test class, the test will automatically skip the async test. This will lead to async test pass in anytime.

an example could be this one:
Under redis instrumentation, I want to add an async test, to test if I run set under a watch process, what will happen. If I just add it under the class we already have, it will pass in any case.

class TestRedis(TestBase):
    def setUp(self):
        super().setUp()
        RedisInstrumentor().instrument(tracer_provider=self.tracer_provider)

    def tearDown(self):
        super().tearDown()
        RedisInstrumentor().uninstrument()

    async def redis_operations(self):
        r = redis.Redis()

        async with r.pipeline(transaction=False) as pipe:
            await pipe.watch("a")

            # Simulate a change by another client
            await r.set("a", "bad")

            pipe.multi()
            await pipe.set("a", "1")

            await pipe.execute()

    @pytest.mark.asyncio
    async def test_redis_operations(self):
        # Run the redis_operations function
        await self.redis_operations()

        # Retrieve the spans
        spans = self.memory_exporter.get_finished_spans()

        # Check that the last span has the correct properties
        assert spans[-1].status.status_code == trace.StatusCode.UNSET

        # Check for specific attributes in the span
        assert any(event.name == "WatchError" for event in spans[-1].events)

Describe the solution you'd like

The solution of test issue is create an async test for it, and the new async test should iInherited from IsolatedAsyncioTestCase
we should add some guide at contributing guide, which could save much time for new rookie contributors(like myself) to debug

Describe alternatives you've considered

not sure if we should add some annotations at test file about this thing

Additional Context

No response

Would you like to implement a fix?

Yes

[emdneto]

@zhihali I think we can have a simple test instruction in https://github.com/open-telemetry/opentelemetry-python-contrib/blob/main/CONTRIBUTING.md#guideline-for-instrumentations . A PR to state this in CONTRIBUTING.md would be nice.

[zhihali]

@zhihali I think we can have a simple test instruction in https://github.com/open-telemetry/opentelemetry-python-contrib/blob/main/CONTRIBUTING.md#guideline-for-instrumentations . A PR to state this in CONTRIBUTING.md would be nice.

Cheers!