AOD: Possibility to inject additional meta data

Commit provides possibility to inject additional (non-hard-coded)
meta data into AOD. This could be used by MC to add information about
timeframe length used or other specific configurations etc.

By default this does not change any production behaviour. Existing
meta data is not touched.

This works by generating a simple key-value json (foo.json) like
```
{
  "ALIEN_JDL_MC_ORBITS_PER_TF": "4",
  "ALIEN_JDL_ANCHOR_SIM_OPTIONS": "-gen pythia8"
}
```

and then exporting an environment variable pointing to this json.

```
export AOD_ADDITIONAL_METADATA_FILE=${PWD}/foo.json
```

Author: sawenzel

Detectors/AOD/src/AODProducerWorkflowSpec.cxx

@@ -102,6 +102,8 @@
 #ifdef WITH_OPENMP
 #include <omp.h>
 #endif
+#include <filesystem>
+#include <nlohmann/json.hpp>
 
 using namespace o2::framework;
 using namespace o2::math_utils::detail;
@@ -1793,6 +1795,38 @@ void AODProducerWorkflowDPL::init(InitContext& ic)
   }
 }
 
+namespace
+{
+void add_additional_meta_info(std::vector<TString>& keys, std::vector<TString>& values)
+{
+  // see if we should put additional meta info (e.g. from MC)
+  auto aod_external_meta_info_file = getenv("AOD_ADDITIONAL_METADATA_FILE");
+  if (aod_external_meta_info_file != nullptr) {
+    LOG(info) << "Trying to inject additional AOD meta-data from " << aod_external_meta_info_file;
+    if (std::filesystem::exists(aod_external_meta_info_file)) {
+      std::ifstream input_file(aod_external_meta_info_file);
+      if (input_file) {
+        nlohmann::json json_data;
+        try {
+          input_file >> json_data;
+        } catch (nlohmann::json::parse_error& e) {
+          std::cerr << "JSON Parse Error: " << e.what() << "\n";
+          std::cerr << "Exception ID: " << e.id << "\n";
+          std::cerr << "Byte position: " << e.byte << "\n";
+          return;
+        }
+        // If parsing succeeds, iterate over key-value pairs
+        for (const auto& [key, value] : json_data.items()) {
+          LOG(info) << "Adding AOD MetaData" << key << " : " << value;
+          keys.push_back(key.c_str());
+          values.push_back(value.get<std::string>());
+        }
+      }
+    }
+  }
+}
+} // namespace
+
 void AODProducerWorkflowDPL::run(ProcessingContext& pc)
 {
   mTimer.Start(false);
@@ -2401,6 +2435,8 @@ void AODProducerWorkflowDPL::run(ProcessingContext& pc)
   TString ROOTVersion = ROOT_RELEASE;
   mMetaDataKeys = {"DataType", "Run", "O2Version", "ROOTVersion", "RecoPassName", "AnchorProduction", "AnchorPassName", "LPMProductionTag", "CreatedBy"};
   mMetaDataVals = {dataType, "3", O2Version, ROOTVersion, mRecoPass, mAnchorProd, mAnchorPass, mLPMProdTag, mUser};
+  add_additional_meta_info(mMetaDataKeys, mMetaDataVals);
+
   pc.outputs().snapshot(Output{"AMD", "AODMetadataKeys", 0}, mMetaDataKeys);
   pc.outputs().snapshot(Output{"AMD", "AODMetadataVals", 0}, mMetaDataVals);