Query: fix exemplarsServer data race (#4777)

Signed-off-by: Jimmiehan <hanjinming@outlook.com>

pkg/exemplars/exemplars.go

@@ -6,6 +6,7 @@ package exemplars
 import (
  "context"
  "sort"
+ "sync"
 
  "github.com/pkg/errors"
  "github.com/prometheus/prometheus/storage"
@@ -37,10 +38,13 @@ type exemplarsServer struct {
 
  warnings []error
  data []*exemplarspb.ExemplarData
+ mu sync.Mutex
 }
 
 func (srv *exemplarsServer) Send(res *exemplarspb.ExemplarsResponse) error {
  if res.GetWarning() != "" {
+ srv.mu.Lock()
+ defer srv.mu.Unlock()
  srv.warnings = append(srv.warnings, errors.New(res.GetWarning()))
  return nil
  }
@@ -49,6 +53,8 @@ func (srv *exemplarsServer) Send(res *exemplarspb.ExemplarsResponse) error {
  return errors.New("empty exemplars data")
  }
 
+ srv.mu.Lock()
+ defer srv.mu.Unlock()
  srv.data = append(srv.data, res.GetData())
  return nil
 }