fix: Generate psedurandom execution id if Function-Execution-Id is not provided; Update README with examples (#260)

* fix: Generate psedurandom execution id if Function-Execution-Id is missing; Update README with exeamples to generate execution Id

* Update README.md to address comments

Author: chujchen

README.md

@@ -133,6 +133,57 @@ handling logic.
 	curl localhost:8080
 	# Output: Hello, World!
 	```
+## Quickstart: Enable Exeuction Id Logging
+
+[Cloud Run Functions(1st gen)](https://cloud.google.com/functions/1stgendocs/deploy) provides an execution id in the logs at `labels.execution_id`, which customers can use to filter their logs for each execution. [Cloud Run Functions](https://cloud.google.com/functions/docs/deploy) doesn't have the same feature embedded. 
+
+To have exeuction id logged for `Cloud Run Functions` executions, users can either:
+
+* Provide a custom execution Id in the Http Header `Function-Execution-Id`.
+
+	```sh
+		curl -H "Function-Execution-Id: 123456" localhost:8080
+		# Output: Hello, World!
+	```
+
+	Example Log:
+	```
+	{"message":"Try logging with executionID!","logging.googleapis.com/labels":{"execution_id":"123456"}}
+	```
+
+
+OR 
+* Leverage `LogWriter` provided in function-framework-go(v1.9.0 or higher) library to generate logs. If `Function-Exeuction-Id` is empty, a pseduorandom execution id will be auto-generated if `LogWriter` is used.  
+
+	```golang
+	package function
+
+	import (
+		"fmt"
+		"net/http"
+		"log"
+		"github.com/GoogleCloudPlatform/functions-framework-go/functions"
+		"github.com/GoogleCloudPlatform/functions-framework-go/funcframework"
+	)
+
+	func init() {
+		functions.HTTP("HelloWorld", helloWorld)
+	}
+
+	// helloWorld writes "Hello, World!" to the HTTP response.
+	func helloWorld(w http.ResponseWriter, r *http.Request) {
+		l := log.New(funcframework.LogWriter(r.Context()), "", 0)
+
+		l.Println("Try logging with executionID!")
+		fmt.Fprintln(w, "Hello, World!")
+	}
+	```
+
+	Example Log:
+	```
+	{"message":"Try logging with executionID!","logging.googleapis.com/labels":{"execution_id":"181dbb5b096549313d470dd68fa64d96"}}
+	```
+
 
 ## Go further: build a deployable container
 

funcframework/logging.go

@@ -4,11 +4,14 @@ import (
 	"bufio"
 	"context"
 	"encoding/json"
+	"fmt"
 	"io"
+	"math/rand"
 	"net/http"
 	"os"
 	"regexp"
 	"sync"
+	"time"
 )
 
 var (
@@ -32,6 +35,11 @@ type contextKey string
 
 func addLoggingIDsToRequest(r *http.Request) *http.Request {
 	executionID := r.Header.Get("Function-Execution-Id")
+	if executionID == "" {
+		timestamp := time.Now().UnixNano()
+		random := rand.Int63()
+		executionID = fmt.Sprintf("%06x%06x", timestamp, random)
+	}
 	traceID, spanID, _ := deconstructXCloudTraceContext(r.Header.Get("X-Cloud-Trace-Context"))
 
 	if executionID == "" && traceID == "" && spanID == "" {
@@ -182,7 +190,7 @@ func (w *structuredLogWriter) Close() error {
 //	)
 //	...
 //	func helloWorld(w http.ResponseWriter, r *http.Request) {
-//	  l := logger.New(funcframework.LogWriter(r.Context()))
+//	  l := log.New(funcframework.LogWriter(r.Context()), "", 0)
 //	  l.Println("hello world!")
 //	}
 func LogWriter(ctx context.Context) io.WriteCloser {

funcframework/logging_test.go

@@ -15,13 +15,15 @@ func TestLoggingIDExtraction(t *testing.T) {
 		wantTraceID     string
 		wantSpanID      string
 		wantExecutionID string
+		randomExecutionIdGenerated	bool
 	}{
 		{
 			name:    "no IDs",
 			headers: map[string]string{},
+			randomExecutionIdGenerated: true,
 		},
 		{
-			name: "execution ID only",
+			name: "provided execution ID only",
 			headers: map[string]string{
 				"Function-Execution-Id": "exec id",
 			},
@@ -32,13 +34,15 @@ func TestLoggingIDExtraction(t *testing.T) {
 			headers: map[string]string{
 				"X-Cloud-Trace-Context": "$*#$(v434)",
 			},
+			randomExecutionIdGenerated: true,
 		},
 		{
 			name: "trace ID only",
 			headers: map[string]string{
 				"X-Cloud-Trace-Context": "0123456789abcdef",
 			},
 			wantTraceID: "0123456789abcdef",
+			randomExecutionIdGenerated: true,
 		},
 		{
 			name: "trace ID and span ID",
@@ -47,6 +51,7 @@ func TestLoggingIDExtraction(t *testing.T) {
 			},
 			wantTraceID: "0123456789abcdef",
 			wantSpanID:  "aaaaaa",
+			randomExecutionIdGenerated: true,
 		},
 		{
 			name: "all",
@@ -77,9 +82,13 @@ func TestLoggingIDExtraction(t *testing.T) {
 				t.Errorf("expected span id %q but got %q", tc.wantSpanID, spid)
 			}
 
-			if eid := ExecutionIDFromContext(ctx); eid != tc.wantExecutionID {
+			eid := ExecutionIDFromContext(ctx); 
+			if tc.wantExecutionID != "" && eid != tc.wantExecutionID {
 				t.Errorf("expected execution id %q but got %q", tc.wantExecutionID, eid)
 			}
+			if tc.randomExecutionIdGenerated && eid == "" {
+				t.Errorf("expected random execution id generated but got %q", eid)
+			}
 		})
 	}
 }