[go-functions] fix metrics server handler error (apache#9394)

### Motivation

apache#9318 add metrics server to go function, but didnt serve `"/"` endpoint in metrics server, which will cause some metrics calls failed.

### Modifications

add handler for `"/"` in `MetricsServicer`

### Verifying this change

- [x] Make sure that the change passes the CI checks.

pulsar-function-go/pf/stats.go

@@ -316,12 +316,14 @@ func (stat *StatWithLabelValues) reset() {
 
 func NewMetricsServicer(goInstance *goInstance) *MetricsServicer {
 	serveMux := http.NewServeMux()
-	serveMux.Handle("/metrics", promhttp.HandlerFor(
+	pHandler := promhttp.HandlerFor(
 		reg,
 		promhttp.HandlerOpts{
 			EnableOpenMetrics: true,
 		},
-	))
+	)
+	serveMux.Handle("/", pHandler)
+	serveMux.Handle("/metrics", pHandler)
 	server := &http.Server{
 		Addr:    fmt.Sprintf(":%d", goInstance.context.GetMetricsPort()),
 		Handler: serveMux,

pulsar-function-go/pf/stats_test.go

@@ -25,6 +25,7 @@ import (
 	"math"
 	"net/http"
 	"testing"
+	"time"
 
 	"github.com/golang/protobuf/proto"
 	"github.com/prometheus/client_golang/prometheus"
@@ -193,12 +194,23 @@ func TestMetricsServer(t *testing.T) {
 	metricsServicer := NewMetricsServicer(gi)
 	metricsServicer.serve()
 	gi.stats.incrTotalReceived()
+	time.Sleep(time.Second * 1)
 
-	resp, err := http.Get(fmt.Sprintf("http://localhost:%d/metrics", gi.context.GetMetricsPort()))
+	resp, err := http.Get(fmt.Sprintf("http://localhost:%d/", gi.context.GetMetricsPort()))
 	assert.Equal(t, nil, err)
+	assert.NotEqual(t, nil, resp)
 	assert.Equal(t, 200, resp.StatusCode)
 	body, err := ioutil.ReadAll(resp.Body)
 	assert.Equal(t, nil, err)
 	assert.NotEmpty(t, body)
 	resp.Body.Close()
+
+	resp, err = http.Get(fmt.Sprintf("http://localhost:%d/metrics", gi.context.GetMetricsPort()))
+	assert.Equal(t, nil, err)
+	assert.NotEqual(t, nil, resp)
+	assert.Equal(t, 200, resp.StatusCode)
+	body, err = ioutil.ReadAll(resp.Body)
+	assert.Equal(t, nil, err)
+	assert.NotEmpty(t, body)
+	resp.Body.Close()
 }