[BUILD] Fix elasticsearch exporter json compatibility (open-telemetry#3313)

* fix build issue with elasticsearch exporter and older versions of nlohmann-json

* run the cmake.test on 22.04 to verify compatibility with the older json package

* use class instead of typename in template args

---------

Author: dbarker

.github/workflows/ci.yml

@@ -41,23 +41,19 @@ jobs:
 #        ./ci/do_ci.sh cmake.exporter.otprotocol.test
 
   cmake_test:
-    name: CMake test (without otlp-exporter)
-    runs-on: ubuntu-latest
+    name: CMake test (prometheus, elasticsearch, zipkin)
+    runs-on: ubuntu-22.04
+    env:
+      CXX_STANDARD: '17'
     steps:
     - uses: actions/checkout@v4
       with:
         submodules: 'recursive'
     - name: setup
-      env:
-        CC: /usr/bin/gcc-12
-        CXX: /usr/bin/g++-12
       run: |
         sudo -E ./ci/setup_googletest.sh
         sudo -E ./ci/setup_ci_environment.sh
-    - name: run cmake tests (without otlp-exporter)
-      env:
-        CC: /usr/bin/gcc-12
-        CXX: /usr/bin/g++-12
+    - name: run cmake tests
       run: |
         ./ci/do_ci.sh cmake.test
 

exporters/elasticsearch/src/es_log_recordable.cc

@@ -34,6 +34,16 @@ struct json_assign_visitor
   {
     *j_ = u;
   }
+
+  template <class U>
+  void operator()(const opentelemetry::nostd::span<U> &span)
+  {
+    *j_ = nlohmann::json::array();
+    for (const auto &elem : span)
+    {
+      j_->push_back(elem);
+    }
+  }
 };
 
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