[SDK] enable deriving from ResourceDetector to create a Resource (#3247)

Author: dbarker

CHANGELOG.md

@@ -30,6 +30,9 @@ Increment the:
 * [SDK] Better control of threads executed by opentelemetry-cpp
   [#3175](https://github.com/open-telemetry/opentelemetry-cpp/pull/3175)
 
+* [SDK] Enable deriving from ResourceDetector to create a Resource
+  [#3247](https://github.com/open-telemetry/opentelemetry-cpp/pull/3247)
+
 New features:
 
 * [SDK] Better control of threads executed by opentelemetry-cpp

sdk/include/opentelemetry/sdk/resource/resource.h

@@ -75,7 +75,7 @@ class Resource
   ResourceAttributes attributes_;
   std::string schema_url_;
 
-  friend class OTELResourceDetector;
+  friend class ResourceDetector;
 };
 
 }  // namespace resource

sdk/include/opentelemetry/sdk/resource/resource_detector.h

@@ -21,6 +21,10 @@ class ResourceDetector
   ResourceDetector()          = default;
   virtual ~ResourceDetector() = default;
   virtual Resource Detect()   = 0;
+
+protected:
+  static Resource Create(const ResourceAttributes &attributes,
+                         const std::string &schema_url = std::string{});
 };
 
 /**

sdk/src/resource/resource_detector.cc

@@ -22,6 +22,12 @@ namespace resource
 const char *OTEL_RESOURCE_ATTRIBUTES = "OTEL_RESOURCE_ATTRIBUTES";
 const char *OTEL_SERVICE_NAME        = "OTEL_SERVICE_NAME";
 
+Resource ResourceDetector::Create(const ResourceAttributes &attributes,
+                                  const std::string &schema_url)
+{
+  return Resource(attributes, schema_url);
+}
+
 Resource OTELResourceDetector::Detect() noexcept
 {
   std::string attributes_str, service_name;
@@ -33,7 +39,7 @@ Resource OTELResourceDetector::Detect() noexcept
 
   if (!attributes_exists && !service_name_exists)
   {
-    return Resource();
+    return ResourceDetector::Create({});
   }
 
   ResourceAttributes attributes;
@@ -59,7 +65,7 @@ Resource OTELResourceDetector::Detect() noexcept
     attributes[semconv::service::kServiceName] = service_name;
   }
 
-  return Resource(attributes);
+  return ResourceDetector::Create(attributes);
 }
 
 }  // namespace resource

sdk/test/resource/resource_test.cc

@@ -35,6 +35,15 @@ class TestResource : public Resource
   {}
 };
 
+class TestResourceDetector : public ResourceDetector
+{
+public:
+  TestResourceDetector() = default;
+  Resource Detect() noexcept override { return Create(attributes, schema_url); }
+  ResourceAttributes attributes;
+  std::string schema_url;
+};
+
 TEST(ResourceTest, create_without_servicename)
 {
   ResourceAttributes expected_attributes = {
@@ -266,4 +275,19 @@ TEST(ResourceTest, OtelResourceDetectorEmptyEnv)
   }
   EXPECT_EQ(received_attributes.size(), expected_attributes.size());
 }
+
 #endif
+
+TEST(ResourceTest, DerivedResourceDetector)
+{
+  TestResourceDetector detector;
+
+  detector.attributes            = {{"key", "value"}};
+  detector.schema_url            = "https://opentelemetry.io/schemas/v3.1.4";
+  const auto resource            = detector.Detect();
+  const auto received_attributes = resource.GetAttributes();
+
+  EXPECT_EQ(received_attributes.size(), 1);
+  EXPECT_EQ(resource.GetSchemaURL(), detector.schema_url);
+  EXPECT_TRUE(received_attributes.find("key") != received_attributes.end());
+}