[EXPORTER] Export resource for prometheus

CHANGELOG.md

@@ -17,6 +17,8 @@ Increment the:
 
 * [DEPRECATION] Deprecate ZPAGES
   [#2291](https://github.com/open-telemetry/opentelemetry-cpp/pull/2291)
+* [EXPORTER] Prometheus exporter emit resource attributes
+  [#2301](https://github.com/open-telemetry/opentelemetry-cpp/pull/2301)
 * [EXPORTER] Remove explicit timestamps from metric points exported by Prometheus
   [#2324](https://github.com/open-telemetry/opentelemetry-cpp/pull/2324)
 * [EXPORTER] Handle attribute key collisions caused by sanitation

exporters/prometheus/include/opentelemetry/exporters/prometheus/collector.h

@@ -31,7 +31,7 @@ class PrometheusCollector : public prometheus_client::Collectable
    * This constructor initializes the collection for metrics to export
    * in this class with default capacity
    */
-  explicit PrometheusCollector(sdk::metrics::MetricReader *reader);
+  explicit PrometheusCollector(sdk::metrics::MetricReader *reader, bool populate_target_info);
 
   /**
    * Collects all metrics data from metricsToCollect collection.
@@ -42,6 +42,7 @@ class PrometheusCollector : public prometheus_client::Collectable
 
 private:
   sdk::metrics::MetricReader *reader_;
+  bool populate_target_info_;
 
   /*
    * Lock when operating the metricsToCollect collection

exporters/prometheus/include/opentelemetry/exporters/prometheus/exporter_options.h

@@ -22,6 +22,9 @@ struct PrometheusExporterOptions
 
   // The endpoint the Prometheus backend can collect metrics from
   std::string url;
+
+  // Populating target_info
+  bool populate_target_info = true;
 };
 
 }  // namespace metrics

exporters/prometheus/include/opentelemetry/exporters/prometheus/exporter_utils.h

@@ -26,12 +26,25 @@ class PrometheusExporterUtils
    * to Prometheus metrics data collection
    *
    * @param records a collection of metrics in OpenTelemetry
+   * @param populate_target_info whether to populate target_info
    * @return a collection of translated metrics that is acceptable by Prometheus
    */
   static std::vector<::prometheus::MetricFamily> TranslateToPrometheus(
-      const sdk::metrics::ResourceMetrics &data);
+      const sdk::metrics::ResourceMetrics &data,
+      bool populate_target_info = true);
 
 private:
+  /**
+   * Append key-value pair to prometheus labels.
+   *
+   * @param name label name
+   * @param value label value
+   * @param labels target labels
+   */
+  static void AddPrometheusLabel(std::string name,
+                                 std::string value,
+                                 std::vector<::prometheus::ClientMetric::Label> *labels);
+
   static opentelemetry::sdk::metrics::AggregationType getAggregationType(
       const opentelemetry::sdk::metrics::PointType &point_type);
 
@@ -41,6 +54,12 @@ class PrometheusExporterUtils
   static ::prometheus::MetricType TranslateType(opentelemetry::sdk::metrics::AggregationType kind,
                                                 bool is_monotonic = true);
 
+  /**
+   * Add a target_info metric to collect resource attributes
+   */
+  static void SetTarget(const sdk::metrics::ResourceMetrics &data,
+                        std::vector<::prometheus::MetricFamily> *output);
+
   /**
    * Set metric data for:
    * Counter => Prometheus Counter
@@ -49,7 +68,8 @@ class PrometheusExporterUtils
   static void SetData(std::vector<T> values,
                       const opentelemetry::sdk::metrics::PointAttributes &labels,
                       ::prometheus::MetricType type,
-                      ::prometheus::MetricFamily *metric_family);
+                      ::prometheus::MetricFamily *metric_family,
+                      const opentelemetry::sdk::resource::Resource *resource);
 
   /**
    * Set metric data for:
@@ -60,13 +80,15 @@ class PrometheusExporterUtils
                       const std::vector<double> &boundaries,
                       const std::vector<uint64_t> &counts,
                       const opentelemetry::sdk::metrics::PointAttributes &labels,
-                      ::prometheus::MetricFamily *metric_family);
+                      ::prometheus::MetricFamily *metric_family,
+                      const opentelemetry::sdk::resource::Resource *resource);
 
   /**
    * Set time and labels to metric data
    */
   static void SetMetricBasic(::prometheus::ClientMetric &metric,
-                             const opentelemetry::sdk::metrics::PointAttributes &labels);
+                             const opentelemetry::sdk::metrics::PointAttributes &labels,
+                             const opentelemetry::sdk::resource::Resource *resource);
 
   /**
    * Convert attribute value to string

exporters/prometheus/src/collector.cc

@@ -17,7 +17,10 @@ namespace metrics
  * This constructor initializes the collection for metrics to export
  * in this class with default capacity
  */
-PrometheusCollector::PrometheusCollector(sdk::metrics::MetricReader *reader) : reader_(reader) {}
+PrometheusCollector::PrometheusCollector(sdk::metrics::MetricReader *reader,
+                                         bool populate_target_info)
+    : reader_(reader), populate_target_info_(populate_target_info)
+{}
 
 /**
  * Collects all metrics data from metricsToCollect collection.
@@ -36,8 +39,19 @@ std::vector<prometheus_client::MetricFamily> PrometheusCollector::Collect() cons
   collection_lock_.lock();
 
   std::vector<prometheus_client::MetricFamily> result;
-  reader_->Collect([&result](sdk::metrics::ResourceMetrics &metric_data) {
-    auto prometheus_metric_data = PrometheusExporterUtils::TranslateToPrometheus(metric_data);
+
+  // If populate_target_info_ is true, populate target_info once.
+  bool populate_target_info = false;
+  if (populate_target_info_)
+  {
+    populate_target_info = true;
+
+    const_cast<PrometheusCollector *>(this)->populate_target_info_ = false;
+  }
+
+  reader_->Collect([&result, populate_target_info](sdk::metrics::ResourceMetrics &metric_data) {
+    auto prometheus_metric_data =
+        PrometheusExporterUtils::TranslateToPrometheus(metric_data, populate_target_info);
     for (auto &data : prometheus_metric_data)
       result.emplace_back(data);
     return true;

exporters/prometheus/src/exporter.cc

@@ -30,7 +30,8 @@ PrometheusExporter::PrometheusExporter(const PrometheusExporterOptions &options)
     Shutdown();  // set MetricReader in shutdown state.
     return;
   }
-  collector_ = std::shared_ptr<PrometheusCollector>(new PrometheusCollector(this));
+  collector_ = std::shared_ptr<PrometheusCollector>(
+      new PrometheusCollector(this, options_.populate_target_info));
 
   exposer_->RegisterCollectable(collector_);
 }