fix(spanner): Expose Mutations field in MutationGroup (#8923)

* fix(spanner): Expose Mutations field in MutationGroup

* Update spanner/mutation.go

Co-authored-by: Sri Harsha CH <57220027+harshachinta@users.noreply.github.com>

---------

Co-authored-by: Sri Harsha CH <57220027+harshachinta@users.noreply.github.com>

spanner/integration_test.go

@@ -924,7 +924,7 @@ func TestIntegration_BatchWrite(t *testing.T) {
 			w.row)
 		ms := make([]*Mutation, 1)
 		ms[0] = m
-		mgs[i] = &MutationGroup{mutations: ms}
+		mgs[i] = &MutationGroup{Mutations: ms}
 	}
 	// Records the mutation group indexes received in the response.
 	seen := make(map[int32]int32)

spanner/mutation.go

@@ -143,8 +143,8 @@ type Mutation struct {
 
 // A MutationGroup is a list of Mutation to be committed atomically.
 type MutationGroup struct {
-	// The mutations in this group
-	mutations []*Mutation
+	// The Mutations in this group
+	Mutations []*Mutation
 }
 
 // mapToMutationParams converts Go map into mutation parameters.
@@ -444,7 +444,7 @@ func mutationsProto(ms []*Mutation) ([]*sppb.Mutation, error) {
 func mutationGroupsProto(mgs []*MutationGroup) ([]*sppb.BatchWriteRequest_MutationGroup, error) {
 	gs := make([]*sppb.BatchWriteRequest_MutationGroup, 0, len(mgs))
 	for _, mg := range mgs {
-		ms, err := mutationsProto(mg.mutations)
+		ms, err := mutationsProto(mg.Mutations)
 		if err != nil {
 			return nil, err
 		}