Fix incorrect row count in EXPLAIN ANALYZE INSERT .. ON CONFLICT output

INSERT ... ON CONFLICT statements record few metrics in the ModifyTable
node's instrument but they get overwritten by hypertable_modify_explain
causing wrong output in EXPLAIN ANALYZE statments. Fix it by saving the
metrics into HypertableModify node before replacing them.

Fixes timescale#6014

.unreleased/bugfix_6014

@@ -0,0 +1 @@
+Fixes: #6014 Correct row count in EXPLAIN ANALYZE INSERT .. ON CONFLICT output

src/nodes/hypertable_modify.c

@@ -227,9 +227,18 @@ hypertable_modify_explain(CustomScanState *node, List *ancestors, ExplainState *
 	}
 #endif
 	/*
-	 * Since we hijack the ModifyTable node instrumentation on ModifyTable will
+	 * Since we hijack the ModifyTable node, instrumentation on ModifyTable will
 	 * be missing so we set it to instrumentation of HypertableModify node.
 	 */
+	if (mtstate->ps.instrument)
+	{
+		/*
+		 * INSERT .. ON CONFLICT statements record few metrics in the ModifyTable node.
+		 * So, copy them into HypertableModify node before replacing them.
+		 */
+		node->ss.ps.instrument->ntuples2 = mtstate->ps.instrument->ntuples2;
+		node->ss.ps.instrument->nfiltered1 = mtstate->ps.instrument->nfiltered1;
+	}
 	mtstate->ps.instrument = node->ss.ps.instrument;
 #endif