out_stackdriver: Add Special field spanId

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

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

Author: 0Delta

pipeline/outputs/stackdriver_special_fields.md

@@ -11,6 +11,7 @@ Currently, we also support some special fields in fluent-bit:
 | 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/trace | trace | Resource name of the trace associated with the log entry |
+| 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 |
 
@@ -324,11 +325,21 @@ The JSON representation is as followed:
 "trace": string
 ```
 
+## 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/trace": "0123456789abcdef0123456789abcdef"
+    "logging.googleapis.com/spanId": "1234567890abcdef"
     ...
 }
 ```
@@ -339,6 +350,7 @@ the logEntry will be:
         ...
     }
     "trace": "projects/your-project-name/traces/0123456789abcdef0123456789abcdef"
+    "spanId": "1234567890abcdef"
     ...
 }
 ```