Add Prometheus metrics relabeling (#3734)

Co-authored-by: Guillermo Ruffino <glm.net@gmail.com>

esphome/components/prometheus/prometheus_handler.h

@@ -2,6 +2,9 @@
 
 #ifdef USE_ARDUINO
 
+#include <map>
+#include <utility>
+
 #include "esphome/components/web_server_base/web_server_base.h"
 #include "esphome/core/controller.h"
 #include "esphome/core/component.h"
@@ -20,6 +23,20 @@ class PrometheusHandler : public AsyncWebHandler, public Component {
    */
   void set_include_internal(bool include_internal) { include_internal_ = include_internal; }
 
+  /** Add the value for an entity's "id" label.
+   *
+   * @param obj The entity for which to set the "id" label
+   * @param value The value for the "id" label
+   */
+  void add_label_id(EntityBase *obj, const std::string &value) { relabel_map_id_.insert({obj, value}); }
+
+  /** Add the value for an entity's "name" label.
+   *
+   * @param obj The entity for which to set the "name" label
+   * @param value The value for the "name" label
+   */
+  void add_label_name(EntityBase *obj, const std::string &value) { relabel_map_name_.insert({obj, value}); }
+
   bool canHandle(AsyncWebServerRequest *request) override {
     if (request->method() == HTTP_GET) {
       if (request->url() == "/metrics")
@@ -41,6 +58,9 @@ class PrometheusHandler : public AsyncWebHandler, public Component {
   }
 
  protected:
+  std::string relabel_id_(EntityBase *obj);
+  std::string relabel_name_(EntityBase *obj);
+
 #ifdef USE_SENSOR
   /// Return the type for prometheus
   void sensor_type_(AsyncResponseStream *stream);
@@ -92,6 +112,8 @@ class PrometheusHandler : public AsyncWebHandler, public Component {
 
   web_server_base::WebServerBase *base_;
   bool include_internal_{false};
+  std::map<EntityBase *, std::string> relabel_map_id_;
+  std::map<EntityBase *, std::string> relabel_map_name_;
 };
 
 }  // namespace prometheus