Fix GKE Emitter with Structured Tags

Fix a bug where only the first structured tag would be emitted in the
gke emitter.

emitter/gkelog/emitter.go

@@ -379,7 +379,7 @@ func Emitter(opt ...Option) alog.Emitter {
 		}
 		for i, sTag := range e.STags {
 			_, asStringTag := tagPositions[sTag.Key] // has this already been used as a string tag?
-			if tagPositions[sTag.Key] != i || asStringTag || reservedKeys[sTag.Key] {
+			if sTagPositions[sTag.Key] != i || asStringTag || reservedKeys[sTag.Key] {
 				continue
 			}
 

emitter/gkelog/emitter_test.go

@@ -57,11 +57,21 @@ func TestLabels(t *testing.T) {
 			X: 1,
 		},
 	}
+
+	structured2 := alog.STag{
+		Key: "structured-again",
+		Val: struct {
+			Y string `json:"val"`
+		}{
+			Y: "foo",
+		},
+	}
+
 	ctx = alog.AddTags(ctx, "allthese", "tags", "andanother", "tag")
-	ctx = alog.AddStructuredTags(ctx, structured)
+	ctx = alog.AddStructuredTags(ctx, structured, structured2)
 	l.Print(ctx, "test")
 
-	want := `{"time":"0001-01-01T00:00:00Z", "allthese":"tags", "andanother":"tag", "structured":{"x":1}, "message":"test"}` + "\n"
+	want := `{"time":"0001-01-01T00:00:00Z", "allthese":"tags", "andanother":"tag", "structured":{"x":1}, "structured-again":{"val":"foo"}, "message":"test"}` + "\n"
 	got := b.String()
 	if got != want {
 		t.Errorf("got:\n%s\nwant:\n%s", got, want)