SERVER-19758 add support for "executionStats" and "allPlansExecution" to agg explain

Like other explainable commands, aggregate can now be
explained using the explain command, e.g.
db.runCommand({explain: {aggregate: ...}, verbosity:
"executionStats"}). The existing explain:true flag
corresponds to "queryPlanner" mode and is still supported.
However, explain:true cannot be specified when explaining
aggregate via the explain command.

Additional execution information is provided only in the
$cursor section of the aggregation explain output. Having
aggregation stages themselves track and report execution
info is further work.

1 files changed, 4 insertions, 2 deletions

diff --git a/src/mongo/db/pipeline/document_source_graph_lookup.h b/src/mongo/db/pipeline/document_source_graph_lookup.h
index 6e94e98b4af..8c14a2ab8fa 100644
--- a/src/mongo/db/pipeline/document_source_graph_lookup.h
+++ b/src/mongo/db/pipeline/document_source_graph_lookup.h
@@ -45,7 +45,9 @@ public:
     const char* getSourceName() const final;
     void dispose() final;
     BSONObjSet getOutputSorts() final;
-    void serializeToArray(std::vector<Value>& array, bool explain = false) const final;
+    void serializeToArray(
+        std::vector<Value>& array,
+        boost::optional<ExplainOptions::Verbosity> explain = boost::none) const final;
 
     /**
      * Returns the 'as' path, and possibly the fields modified by an absorbed $unwind.
@@ -107,7 +109,7 @@ private:
         boost::optional<long long> maxDepth,
         boost::optional<boost::intrusive_ptr<DocumentSourceUnwind>> unwindSrc);
 
-    Value serialize(bool explain = false) const final {
+    Value serialize(boost::optional<ExplainOptions::Verbosity> explain = boost::none) const final {
         // Should not be called; use serializeToArray instead.
         MONGO_UNREACHABLE;
     }