Align SDK with resource detection spec

[aabmass]

Resource detection has been merged into the spec. The SDK should be aligned with the spec. This may already be the case.

Is your feature request related to a problem?
No

Describe the solution you'd like
The resource detection spec is flexible and doesn't prescribe anything. Currently, we have this code:

opentelemetry-python/opentelemetry-sdk/src/opentelemetry/sdk/resources/__init__.py

Lines 65 to 120 in c435600

	class ResourceDetector(abc.ABC):
	def __init__(self, raise_on_error=False):
	self.raise_on_error = raise_on_error
	@abc.abstractmethod
	def detect(self) -> "Resource":
	raise NotImplementedError()
	class OTELResourceDetector(ResourceDetector):
	# pylint: disable=no-self-use
	def detect(self) -> "Resource":
	env_resources_items = os.environ.get("OTEL_RESOURCE_ATTRIBUTES")
	env_resource_map = {}
	if env_resources_items:
	env_resource_map = {
	key.strip(): value.strip()
	for key, value in (
	item.split("=") for item in env_resources_items.split(",")
	)
	}
	return Resource(env_resource_map)
	def get_aggregated_resources(
	detectors: typing.List["ResourceDetector"],
	initial_resource: typing.Optional[Resource] = None,
	timeout=5,
	) -> "Resource":
	""" Retrieves resources from detectors in the order that they were passed
	:param detectors: List of resources in order of priority
	:param initial_resource: Static resource. This has highest priority
	:param timeout: Number of seconds to wait for each detector to return
	:return:
	"""
	final_resource = initial_resource or _EMPTY_RESOURCE
	detectors = [OTELResourceDetector()] + detectors
	with concurrent.futures.ThreadPoolExecutor(max_workers=4) as executor:
	futures = [executor.submit(detector.detect) for detector in detectors]
	for detector_ind, future in enumerate(futures):
	detector = detectors[detector_ind]
	try:
	detected_resources = future.result(timeout=timeout)
	# pylint: disable=broad-except
	except Exception as ex:
	if detector.raise_on_error:
	raise ex
	logger.warning(
	"Exception %s in detector %s, ignoring", ex, detector
	)
	detected_resources = _EMPTY_RESOURCE
	finally:
	final_resource = final_resource.merge(detected_resources)
	return final_resource

We could:

• Leave everything as it is. The spec doesn't say we need a ResourceDetector interface or get_aggregated_resources() nor does it say it shouldn't be there.
• Remove the ResourceDetector interface and get_aggregated_resources()
• Replace the ResourceDetector interface with a Callable[[bool], Resource] function param, since the spec only says

  Resource detector packages MUST provide a method that returns a resource.

Thoughts?

[lzchen]

I like having the ResourceDetector interface, it gives users an idea of how to implement their own detector.
Is get_aggregated_resources() something we expect users to call explictly? As a user, I'd expect to just instantiate my own ResourceDetector (s) and pass them into the TracerProvider. So instead of doing something like this, maybe we can change TracerProvider to accept a sequence of ResoureDetectors, and then we merge them upon instantiation of the provider underneath (so we don't have to expose get_aggregated_resources() to the user.

[lzchen]

The Callable[[bool], Resource] is a cool idea. I'd think they would have to depend on the SDK since they would be returning a Resource?

[aabmass]

I'd think they would have to depend on the SDK since they would be returning a Resource?

Oh ya, my bad. Updated the issue description.

maybe we can change TracerProvider to accept a sequence of ResoureDetectors, and then we merge them upon instantiation of the provider underneath

I like this interface better, not sure if that would be taking too much liberty interpreting the spec though. I think having a util to run the detectors in parallel is worth having, once way or another.

[codeboten]

As discussed in the SIG meeting get_aggregated_resources can stay but the recommendation is to pass in merged resources, for which users can use get_aggregated_resources

[aabmass]

Cool, so the only thing remaining is calling and using the OTELResourceDetector in the providers