Add span content for preview result before use.

Signed-off-by: helloworlde <hellowoodes@gmail.com>

Author: helloworlde

README.md

@@ -11,7 +11,8 @@ package main
 
 import (
     "context"
-    "fmt"
+	"encoding/json"
+	"fmt"
     "strconv"
 
     "github.com/tikv/minitrace-go"
@@ -50,7 +51,108 @@ func main() {
     // collect tracing results into `spans`
     spans, _ := root.Collect()
 
-    // do something with `spans`
-    fmt.Printf("%+v", spans)
+    // print `spans` content in JSON format
+	spanContent, _ := json.MarshalIndent(spans, "", "  ")
+	fmt.Println(string(spanContent))
+}
+```
+
+The output of spans in JSON format like this:
+
+```json
+{
+  "TraceID": 10010,
+  "Spans": [
+    {
+      "ID": 15352856648520921629,
+      "ParentID": 8674665223082153551,
+      "BeginUnixTimeNs": 1662446668306972169,
+      "DurationNs": 87,
+      "Event": "2",
+      "Properties": null
+    },
+    {
+      "ID": 13260572831089785859,
+      "ParentID": 8674665223082153551,
+      "BeginUnixTimeNs": 1662446668306973148,
+      "DurationNs": 61,
+      "Event": "3",
+      "Properties": null
+    },
+    {
+      "ID": 3916589616287113937,
+      "ParentID": 8674665223082153551,
+      "BeginUnixTimeNs": 1662446668306979536,
+      "DurationNs": 63,
+      "Event": "4",
+      "Properties": null
+    },
+    {
+      "ID": 6334824724549167320,
+      "ParentID": 8674665223082153551,
+      "BeginUnixTimeNs": 1662446668306983182,
+      "DurationNs": 63,
+      "Event": "5",
+      "Properties": null
+    },
+    {
+      "ID": 9828766684487745566,
+      "ParentID": 8674665223082153551,
+      "BeginUnixTimeNs": 1662446668306983484,
+      "DurationNs": 59,
+      "Event": "6",
+      "Properties": null
+    },
+    {
+      "ID": 10667007354186551956,
+      "ParentID": 8674665223082153551,
+      "BeginUnixTimeNs": 1662446668306987321,
+      "DurationNs": 61,
+      "Event": "7",
+      "Properties": null
+    },
+    {
+      "ID": 894385949183117216,
+      "ParentID": 8674665223082153551,
+      "BeginUnixTimeNs": 1662446668306987618,
+      "DurationNs": 61,
+      "Event": "8",
+      "Properties": null
+    },
+    {
+      "ID": 11998794077335055257,
+      "ParentID": 8674665223082153551,
+      "BeginUnixTimeNs": 1662446668306987898,
+      "DurationNs": 60,
+      "Event": "9",
+      "Properties": null
+    },
+    {
+      "ID": 8674665223082153551,
+      "ParentID": 5577006791947779410,
+      "BeginUnixTimeNs": 1662446668306967492,
+      "DurationNs": 20570,
+      "Event": "1",
+      "Properties": [
+        {
+          "Key": "k2",
+          "Value": "v2"
+        }
+      ]
+    },
+    {
+      "ID": 5577006791947779410,
+      "ParentID": 0,
+      "BeginUnixTimeNs": 1662446668306966615,
+      "DurationNs": 24319,
+      "Event": "root",
+      "Properties": [
+        {
+          "Key": "k1",
+          "Value": "v1"
+        }
+      ]
+    }
+  ]
 }
 ```