Add EntityRef and use it in Resource. (open-telemetry#635)

* Add EntityRef and use it in Resource.

* Update opentelemetry/proto/resource/v1/resource.proto

Co-authored-by: Nathan L Smith <nathan.smith@elastic.co>

* Update opentelemetry/proto/common/v1/common.proto

Co-authored-by: Nathan L Smith <nathan.smith@elastic.co>

* Update opentelemetry/proto/common/v1/common.proto

Co-authored-by: Nathan L Smith <nathan.smith@elastic.co>

---------

Co-authored-by: Nathan L Smith <nathan.smith@elastic.co>

Author: jsuereth

opentelemetry/proto/common/v1/common.proto

@@ -79,3 +79,37 @@ message InstrumentationScope {
   repeated KeyValue attributes = 3;
   uint32 dropped_attributes_count = 4;
 }
+
+// A reference to an Entity.
+// Entity represents an object of interest associated with produced telemetry: e.g spans, metrics, profiles, or logs.
+//
+// Status: [Development]
+message EntityRef {
+  // The Schema URL, if known. This is the identifier of the Schema that the entity data
+  // is recorded in. To learn more about Schema URL see
+  // https://opentelemetry.io/docs/specs/otel/schemas/#schema-url
+  //
+  // This schema_url applies to the data in this message and to the Resource attributes
+  // referenced by id_keys and description_keys.
+  // TODO: discuss if we are happy with this somewhat complicated definition of what
+  // the schema_url applies to.
+  //
+  // This field obsoletes the schema_url field in ResourceMetrics/ResourceSpans/ResourceLogs.
+  string schema_url = 1;
+
+  // Defines the type of the entity. MUST not change during the lifetime of the entity.
+  // For example: "service" or "host". This field is required and MUST not be empty
+  // for valid entities.
+  string type = 2;
+
+  // Attribute Keys that identify the entity.
+  // MUST not change during the lifetime of the entity. The Id must contain at least one attribute.
+  // These keys MUST exist in the containing {message}.attributes.
+  repeated string id_keys = 3;
+
+  // Descriptive (non-identifying) attribute keys of the entity.
+  // MAY change over the lifetime of the entity. MAY be empty.
+  // These attribute keys are not part of entity's identity.
+  // These keys MUST exist in the containing {message}.attributes.
+  repeated string description_keys = 4;
+}

opentelemetry/proto/resource/v1/resource.proto

@@ -34,4 +34,11 @@ message Resource {
   // dropped_attributes_count is the number of dropped attributes. If the value is 0, then
   // no attributes were dropped.
   uint32 dropped_attributes_count = 2;
+
+  // Set of entities that participate in this Resource.
+  //
+  // Note: keys in the references MUST exist in attributes of this message.
+  //
+  // Status: [Development]
+  repeated opentelemetry.proto.common.v1.EntityRef entity_refs = 3;
 }