out_stackdriver: Add Special field spanId

Ref: [fluent-bit #4224](fluent/fluent-bit#4224)

Signed-off-by: 0Delta <[email protected]>

pipeline/outputs/stackdriver_special_fields.md

@@ -10,6 +10,7 @@ Currently, we also support some special fields in fluent-bit:
 | logging.googleapis.com/insertId | insertId | A unique identifier for the log entry. It is used to order logEntries |
 | logging.googleapis.com/sourceLocation | sourceLocation | Additional information about the source code location that produced the log entry. |
 | logging.googleapis.com/http_request | httpRequest | A common proto for logging HTTP requests. |
+| logging.googleapis.com/spanId | spanId | The span ID within the trace associated with the log entry |
 | timestamp | timestamp | An object including the seconds and nanos fields that represents the time |
 | timestampSecond & timestampNanos | timestamp | The seconds and nanos that represents the time |
 
@@ -313,6 +314,34 @@ the logEntry will be:
 }
 ```
 
+## Span ID
+
+The span ID within the trace associated with the log entry.
+
+The JSON representation is as followed:
+```text
+"spanId": string
+```
+
+### Use Cases
+Set the input log as followed:
+```text
+jsonPayload {
+    "logging.googleapis.com/spanId": "1234567890abcdef"
+    ...
+}
+```
+the logEntry will be:
+```text
+{
+    "jsonPayload": {
+        ...
+    }
+    "spanId": "1234567890abcdef"
+    ...
+}
+```
+
 ## Timestamp
 
 We support two formats of time-related fields: