Make it easier to add a resource resource.Detector to the SDKs

[dashpole]

Make it easier to add a resource resource.Detector to the SDKs

[dashpole]

I was reading through the spec for resource, and it seems like our SDKs aren't compliant with the resource spec (which isn't part of the SDK specification).

The SDK MUST extract information from the OTEL_RESOURCE_ATTRIBUTES environment
variable and merge this, as the secondary resource, with any resource
information provided by the user, i.e. the user provided resource information
has higher priority.

The spec for the default service name has similar language (also not in the SDK spec): https://github.com/open-telemetry/semantic-conventions/blob/61870711f9e7c13c3a058215a15e50877dd92d77/docs/attributes-registry/service.md?plain=1#L47

[service.name] MUST be the same for all instances of horizontally scaled services. If the value was not specified, SDKs MUST fallback to unknown_service: concatenated with process.executable.name, e.g. unknown_service:bash. If process.executable.name is not available, the value MUST be set to unknown_service.

Same for the telemetry detector (also not in the SDK specification): https://github.com/open-telemetry/semantic-conventions/blob/61870711f9e7c13c3a058215a15e50877dd92d77/docs/attributes-registry/telemetry.md

The OpenTelemetry SDK MUST set the telemetry.sdk.name attribute to opentelemetry. If another SDK, like a fork or a vendor-provided implementation, is used, this SDK MUST set the telemetry.sdk.name attribute to the fully-qualified class or module name of this SDK's main entry point or another suitable identifier depending on the language. The identifier opentelemetry is reserved and MUST NOT be used in this case. All custom identifiers SHOULD be stable across different versions of an implementation.

The above seems to imply SDKs MUST include our Default resource, and merge it with the user-provided resource. It seems like the correct resolution here is to not add an additional option, but instead to perform that merge within the SDK initialization. Thoughts?

[MrAlias]

The SDK provides the Default function to comply with this requirement.

There are a few ways create a resource. I'm not sure we want to impose that a default resource be included in all of them by default. It seems like it would impose potentially unnecessary composition on all use-cases.

[MrAlias]

We want to look at other language implementation to determine if we are following what they do for our defaults.

Holding until that work is done.