Do not override span_id with otel value by default

This is done to prevent breaking of behavior for people that have both set and already rely for regular span_id to be logged to DD.

lib/logger_json/formatters/datadog_logger.ex

@@ -72,14 +72,14 @@ defmodule LoggerJSON.Formatters.DatadogLogger do
   # To connect logs and traces, span_id and trace_id keys are respectively dd.span_id and dd.trace_id
   # https://docs.datadoghq.com/tracing/faq/why-cant-i-see-my-correlated-logs-in-the-trace-id-panel/?tab=jsonlogs
   defp convert_tracing_keys(output, md) do
-    fields = %{
-      span_id: ["dd.span_id", & &1],
-      trace_id: ["dd.trace_id", & &1],
-      otel_span_id: ["dd.span_id", &convert_otel_field/1],
-      otel_trace_id: ["dd.trace_id", &convert_otel_field/1]
-    }
-
-    Enum.reduce(fields, output, fn {key, [new_key, transformer]}, acc ->
+    # Notice: transformers can override each others but the last one in this list wins
+    [
+      otel_span_id: {"dd.span_id", &convert_otel_field/1},
+      otel_trace_id: {"dd.trace_id", &convert_otel_field/1},
+      span_id: {"dd.span_id", & &1},
+      trace_id: {"dd.trace_id", & &1}
+    ]
+    |> Enum.reduce(output, fn {key, {new_key, transformer}}, acc ->
       if Keyword.has_key?(md, key) do
         new_value = transformer.(Keyword.get(md, key))
         Map.put(acc, new_key, new_value)

test/unit/logger_json/formatters/datadog_logger_test.exs

@@ -224,6 +224,25 @@ defmodule LoggerJSONDatadogTest do
       assert Map.has_key?(log, "span_id") == false
     end
 
+    test "trace_id/span_id has greater priority than otel_trace_id/otel_span_id" do
+      Logger.configure_backend(LoggerJSON, metadata: :all)
+
+      otel_id =
+        <<98, 56, 49, 48, 100, 98, 97, 50, 57, 56, 48, 51, 101, 101, 54, 49, 101, 55, 99, 55, 49, 102, 102, 48, 99, 50,
+          99, 57, 53, 97, 57, 100>>
+
+      Logger.metadata(otel_trace_id: otel_id, trace_id: "1", span_id: "2", otel_span_id: otel_id)
+
+      log =
+        fn -> Logger.debug("hello") end
+        |> capture_log()
+        |> Jason.decode!()
+
+      assert %{"dd.trace_id" => "1", "dd.span_id" => "2"} = log
+      assert Map.has_key?(log, "trace_id") == false
+      assert Map.has_key?(log, "span_id") == false
+    end
+
     test "convert otel_trace_id/otel_span_id binary to expected datadog keys" do
       Logger.configure_backend(LoggerJSON, metadata: :all)