Define how Schema URL works for Resources

This adds to the Resource SDK the concept of Schema URL proposed in OTEP 0152:
https://github.com/open-telemetry/oteps/blob/main/text/0152-telemetry-schemas.md

Related earlier PR that adds the Schema URL to the API:
#1666

CHANGELOG.md

@@ -18,6 +18,10 @@ release.
 
 ### Logs
 
+### Resource
+
+- Add schema_url support to `Resource`. ([#1692](https://github.com/open-telemetry/opentelemetry-specification/pull/1692))
+
 ### Semantic Conventions
 
 - Add JSON RPC specific conventions ([#1643](https://github.com/open-telemetry/opentelemetry-specification/pull/1643)).

specification/resource/sdk.md

@@ -56,6 +56,9 @@ object. A factory method is recommended to enable support for cached objects.
 Required parameters:
 
 - [`Attributes`](../common/common.md#attributes)
+- [since 1.4.0] `schema_url` (optional): Specifies the Schema URL that should be
+  recorded in the emitted resource. If the `schema_url` parameter is unspecified
+  then the created resource will have an empty Schema URL.
 
 ### Merge
 
@@ -70,6 +73,17 @@ The resulting resource MUST have all attributes that are on any of the two input
 If a key exists on both the old and updating resource, the value of the updating
 resource MUST be picked (even if the updated value is empty).
 
+The resulting resource will have the Schema URL calculated as follows:
+
+- If the old resource's Schema URL is empty then the resulting resource's Schema
+  URL will be set to the Schema URL of the updating resource,
+- Else if the updating resource's Schema URL is empty then the resulting
+  resource's Schema URL will be set to the Schema URL of the old resource,
+- Else if the Schema URLs of the old and updating resources are the same then
+  that will be the Schema URL of the resulting resource,
+- Else this is a merging error (this is the case when the Schema URL of the old
+  and updating resources are not empty and are different).
+
 Required parameters:
 
 - the old resource
@@ -102,6 +116,16 @@ failure to detect any resource information MUST NOT be considered an error,
 whereas an error that occurs during an attempt to detect resource information
 SHOULD be considered an error.
 
+Resource detectors that populate resource attributes according to OpenTelemetry
+semantic conventions MUST ensure that the resource has a Schema URL set to a
+value that matches the semantic conventions. Empty Schema URL SHOULD be used if
+the detector does not populate the resource with any known attributes that have
+a semantic convention or if the detector does not know what attributes it will
+populate (e.g. the detector that reads the attributes from environment values
+will not know what Schema URL to use). If multiple detectors are combined and
+the detectors use different non-empty Schema URL it MUST be an error since it is
+impossible to merge such resources.
+
 ### Specifying resource information via an environment variable
 
 The SDK MUST extract information from the `OTEL_RESOURCE_ATTRIBUTES` environment