Add JsonNode construction benchmarks (#4463)

Add JsonNode construction benchmarks

Author: eiriktsarpalis

src/benchmarks/micro/libraries/System.Text.Json/Node/Perf_Create.cs

@@ -0,0 +1,105 @@
+using BenchmarkDotNet.Attributes;
+using MicroBenchmarks;
+using System.Text.Json.Nodes;
+
+namespace System.Text.Json.Node.Tests
+{
+    [BenchmarkCategory(Categories.Libraries, Categories.JSON)]
+    public class Perf_Create
+    {
+        private readonly JsonNode[] _results = new JsonNode[50];
+
+        [Benchmark]
+        public Span<JsonNode> Create_JsonBool()
+        {
+            Span<JsonNode> results = _results.AsSpan();
+            for (int i = 0; i < results.Length; i++)
+            {
+                results[i] = (JsonNode)true;
+            }
+
+            return results;
+        }
+
+        [Benchmark]
+        public Span<JsonNode> Create_JsonNumber()
+        {
+            Span<JsonNode> results = _results.AsSpan();
+            for (int i = 0; i < results.Length; i++)
+            {
+                results[i] = (JsonNode)42;
+            }
+
+            return results;
+        }
+
+        [Benchmark]
+        public Span<JsonNode> Create_JsonString()
+        {
+            Span<JsonNode> results = _results.AsSpan();
+            for (int i = 0; i < results.Length; i++)
+            {
+                results[i] = (JsonNode)"some string";
+            }
+
+            return results;
+        }
+
+        [Benchmark]
+        public Span<JsonNode> Create_JsonArray()
+        {
+            Span<JsonNode> results = _results.AsSpan(0, 20);
+            for (int i = 0; i < results.Length; i++)
+            {
+                results[i] = new JsonArray { null, null, null, null };
+            }
+
+            return results;
+        }
+
+        [Benchmark]
+        public JsonNode Create_JsonObject_Small()
+        {
+            return new JsonObject
+            {
+                ["prop0"] = null,
+                ["prop1"] = null,
+                ["prop2"] = null,
+                ["prop3"] = null,
+                ["prop4"] = null,
+                ["prop5"] = null,
+                ["prop6"] = null,
+                ["prop7"] = null,
+                ["prop8"] = null,
+            };
+        }
+
+        [Benchmark]
+        public JsonNode Create_JsonObject_Large()
+        {
+            return new JsonObject
+            {
+                ["prop0"] = null,
+                ["prop1"] = null,
+                ["prop2"] = null,
+                ["prop3"] = null,
+                ["prop4"] = null,
+                ["prop5"] = null,
+                ["prop6"] = null,
+                ["prop7"] = null,
+                ["prop8"] = null,
+                ["prop9"] = null,
+                ["prop10"] = null,
+                ["prop11"] = null,
+                ["prop12"] = null,
+                ["prop13"] = null,
+                ["prop14"] = null,
+                ["prop15"] = null,
+                ["prop16"] = null,
+                ["prop17"] = null,
+                ["prop18"] = null,
+                ["prop19"] = null,
+            };
+        }
+    }
+}