[DJM-964] Add Spark physical plan with spark.sql spans

Author: charlesmyu

dd-java-agent/instrumentation/spark/src/main/java/datadog/trace/instrumentation/spark/AbstractDatadogSparkListener.java

@@ -411,6 +411,7 @@ private AgentSpan getOrCreateSqlSpan(
             .withTag("query_id", sqlExecutionId)
             .withTag("description", queryStart.description())
             .withTag("details", queryStart.details())
+            .withTag("_dd.spark.physical_plan", queryStart.physicalPlanDescription())
             .withTag(DDTags.RESOURCE_NAME, queryStart.description());
 
     if (batchKey != null) {