feat(push): Add method to delete metrics for instance

Fixes: #303

Author: gjasny

push/include/prometheus/gateway.h

@@ -52,6 +52,12 @@ class PROMETHEUS_CPP_PUSH_EXPORT Gateway {
   // Delete metrics from the given pushgateway.
   std::future<int> AsyncDelete();
 
+  // Delete metrics from the given pushgateway (for configured instance labels).
+  int DeleteForInstance();
+
+  // Delete metrics from the given pushgateway (for configured instance labels).
+  std::future<int> AsyncDeleteForInstance();
+
  private:
   std::string jobUri_;
   std::string labels_;

push/src/gateway.cc

@@ -146,6 +146,15 @@ std::future<int> Gateway::AsyncDelete() {
   return std::async(std::launch::async, [&] { return Delete(); });
 }
 
+int Gateway::DeleteForInstance() {
+  return curlWrapper_->performHttpRequest(detail::HttpMethod::Delete,
+                                          jobUri_ + labels_, {});
+}
+
+std::future<int> Gateway::AsyncDeleteForInstance() {
+  return std::async(std::launch::async, [&] { return DeleteForInstance(); });
+}
+
 void Gateway::CleanupStalePointers(
     std::vector<CollectableEntry>& collectables) {
   collectables.erase(