Document and check resource comparability

Ensure backwards compatibility by adding a compile-time check that the
Resource remains comparable.

Document the shortcomings of direct comparison of a Resource.

Author: MrAlias

sdk/resource/resource.go

@@ -21,11 +21,20 @@ import (
 // Resources should be passed and stored as pointers
 // (`*resource.Resource`).  The `nil` value is equivalent to an empty
 // Resource.
+//
+// While Resource is comparable, its comparison should not be relied on when
+// checking equality. The [Resource.Equal] method should be used to check
+// equality between two resources and the [attribute.Distinct] returned from
+// [Resource.Equivalent] should be used for map keys instead. No guantee of the
+// correctness of direct comparisons is provided.
 type Resource struct {
 	attrs     attribute.Set
 	schemaURL string
 }
 
+// Compile-time check that the Resource remains comparable.
+var _ map[Resource]struct{} = nil
+
 var (
 	defaultResource     *Resource
 	defaultResourceOnce sync.Once