fix(zero): fix zero's health endpoint to return json response (#8858)

Fixes #5517 Zero's /health endpoint just returns OK unlike Alpha's JSON
output.

## Problem
curl http://alpha-hostname:alpha-http-port/health returns a JSON output
whereas, curl http://zero-hostname:zero-http-port/health returns a plain
text "OK".

## Solution
Added code to return a plain text output ("OK") for the "text/plain" to
maintain the backward compatibility and return JSON output for "Accept:
application/json" header.

Example Output:
$ curl http://localhost:58696/health
OK

$ curl -H "Accept: application/json" http://localhost:58696/health

{"instance":"zero","address":"zero1:5080","status":"healthy","version":"v23.0.0-16-g90139243f","uptime":9223372036,"lastEcho":1686137664}

dgraph/cmd/zero/http.go

@@ -18,6 +18,7 @@ package zero
 
 import (
 	"context"
+	"encoding/json"
 	"fmt"
 	"net/http"
 	"strconv"
@@ -26,7 +27,9 @@ import (
 
 	"github.com/gogo/protobuf/jsonpb"
 	"github.com/golang/glog"
+	"github.com/pkg/errors"
 
+	"github.com/dgraph-io/dgo/v230/protos/api"
 	"github.com/dgraph-io/dgraph/protos/pb"
 	"github.com/dgraph-io/dgraph/x"
 )
@@ -231,9 +234,56 @@ func (st *state) getState(w http.ResponseWriter, r *http.Request) {
 	}
 }
 
+func (s *Server) zeroHealth(ctx context.Context) (*api.Response, error) {
+	if ctx.Err() != nil {
+		return nil, errors.Wrap(ctx.Err(), "http request context error")
+	}
+	health := pb.HealthInfo{
+		Instance: "zero",
+		Address:  x.WorkerConfig.MyAddr,
+		Status:   "healthy",
+		Version:  x.Version(),
+		Uptime:   int64(time.Since(x.WorkerConfig.StartTime) / time.Second),
+		LastEcho: time.Now().Unix(),
+	}
+	jsonOut, err := json.Marshal(health)
+	if err != nil {
+		return nil, errors.Wrapf(err, "unable to marshal zero health, error")
+	}
+	return &api.Response{Json: jsonOut}, nil
+}
+
 func (st *state) pingResponse(w http.ResponseWriter, r *http.Request) {
 	x.AddCorsHeaders(w)
 
-	w.WriteHeader(http.StatusOK)
-	_, _ = w.Write([]byte("OK"))
+	/*
+	 * zero is changed to also output the health in JSON format for client
+	 * request header "Accept: application/json".
+	 *
+	 * Backward compatibility- Before this change the '/health' endpoint
+	 * used to output the string OK. After the fix it returns OK when the
+	 * client sends the request without "Accept: application/json" in its
+	 * http header.
+	 */
+	switch r.Header.Get("Accept") {
+	case "application/json":
+		resp, err := (st.zero).zeroHealth(r.Context())
+		if err != nil {
+			x.SetStatus(w, x.Error, err.Error())
+			return
+		}
+		w.Header().Set("Content-Type", "application/json; charset=utf-8")
+		w.WriteHeader(http.StatusOK)
+		if _, err := w.Write(resp.Json); err != nil {
+			glog.Warningf("http error send failed, error msg=[%v]", err)
+			return
+		}
+	default:
+		w.Header().Set("Content-Type", "text/plain; charset=utf-8")
+		w.WriteHeader(http.StatusOK)
+		if _, err := w.Write([]byte("OK")); err != nil {
+			glog.Warningf("http error send failed, error msg=[%v]", err)
+			return
+		}
+	}
 }

dgraph/cmd/zero/zero_test.go

@@ -20,8 +20,13 @@ package zero
 
 import (
 	"context"
+	"encoding/json"
+	"io/ioutil"
 	"math"
+	"net/http"
+	"net/url"
 	"testing"
+	"time"
 
 	"github.com/stretchr/testify/require"
 	"google.golang.org/grpc"
@@ -112,3 +117,48 @@ func TestProposalKey(t *testing.T) {
 	}
 	require.Equal(t, len(uniqueKeys), 10, "each iteration should create unique key")
 }
+
+func TestZeroHealth(t *testing.T) {
+	client := http.Client{Timeout: 3 * time.Second}
+	u := &url.URL{
+		Scheme: "http",
+		Host:   testutil.ContainerAddr("zero1", 6080),
+		Path:   "health",
+	}
+
+	// JSON format
+	req, err := http.NewRequest("GET", u.String(), nil)
+	require.NoError(t, err)
+
+	req.Header.Add("Accept", `application/json`)
+	start := time.Now().Unix()
+	resp, err := client.Do(req)
+	require.NoError(t, err)
+	defer resp.Body.Close()
+
+	body, err := ioutil.ReadAll(resp.Body)
+	require.NoError(t, err)
+
+	var r map[string]interface{}
+	err = json.Unmarshal(body, &r)
+	require.NoError(t, err)
+
+	require.Equal(t, "zero", r["instance"].(string))
+	require.Equal(t, "zero1:5080", r["address"].(string))
+	require.Equal(t, "healthy", r["status"].(string))
+	require.NotEqual(t, 0, len(r["version"].(string)))
+	require.Greater(t, r["uptime"].(float64), 0.0)
+	require.GreaterOrEqual(t, int64(r["lastEcho"].(float64)), start)
+
+	// String format
+	req, err = http.NewRequest("GET", u.String(), nil)
+	require.NoError(t, err)
+
+	resp, err = client.Do(req)
+	require.NoError(t, err)
+	defer resp.Body.Close()
+
+	body, err = ioutil.ReadAll(resp.Body)
+	require.NoError(t, err)
+	require.Equal(t, string(body), "OK")
+}