filters/tracing: improve stateBagToTag

* reduce allocations when tag value is string
* check statebag value first to bail out early if it is absent
* restrict max number of arguments to two

```
goos: linux
goarch: amd64
pkg: github.com/zalando/skipper/filters/tracing
cpu: Intel(R) Core(TM) i5-8350U CPU @ 1.70GHz
                            │   HEAD~1    │                HEAD                 │
                            │   sec/op    │   sec/op     vs base                │
StateBagToTag_StringValue-8   313.1n ± 2%   124.3n ± 1%  -60.29% (p=0.000 n=10)

                            │   HEAD~1   │                HEAD                │
                            │    B/op    │   B/op     vs base                 │
StateBagToTag_StringValue-8   19.00 ± 0%   0.00 ± 0%  -100.00% (p=0.000 n=10)

                            │   HEAD~1   │                HEAD                 │
                            │ allocs/op  │ allocs/op   vs base                 │
StateBagToTag_StringValue-8   2.000 ± 0%   0.000 ± 0%  -100.00% (p=0.000 n=10)
```

Signed-off-by: Alexander Yastrebov <[email protected]>

filters/tracing/statebagtotag.go

@@ -25,7 +25,7 @@ func (stateBagToTagSpec) Name() string {
 }
 
 func (stateBagToTagSpec) CreateFilter(args []interface{}) (filters.Filter, error) {
-	if len(args) < 1 {
+	if len(args) < 1 || len(args) > 2 {
 		return nil, filters.ErrInvalidFilterParameters
 	}
 
@@ -43,7 +43,7 @@ func (stateBagToTagSpec) CreateFilter(args []interface{}) (filters.Filter, error
 		tagName = tagNameArg
 	}
 
-	return stateBagToTagFilter{
+	return &stateBagToTagFilter{
 		stateBagItemName: stateBagItemName,
 		tagName:          tagName,
 	}, nil
@@ -53,16 +53,22 @@ func NewStateBagToTag() filters.Spec {
 	return stateBagToTagSpec{}
 }
 
-func (f stateBagToTagFilter) Request(ctx filters.FilterContext) {
+func (f *stateBagToTagFilter) Request(ctx filters.FilterContext) {
+	value, ok := ctx.StateBag()[f.stateBagItemName]
+	if !ok {
+		return
+	}
+
 	span := opentracing.SpanFromContext(ctx.Request().Context())
 	if span == nil {
 		return
 	}
-	value, ok := ctx.StateBag()[f.stateBagItemName]
-	if !ok {
-		return
+
+	if _, ok := value.(string); ok {
+		span.SetTag(f.tagName, value)
+	} else {
+		span.SetTag(f.tagName, fmt.Sprint(value))
 	}
-	span.SetTag(f.tagName, fmt.Sprint(value))
 }
 
-func (stateBagToTagFilter) Response(ctx filters.FilterContext) {}
+func (*stateBagToTagFilter) Response(ctx filters.FilterContext) {}

filters/tracing/statebagtotag_test.go

@@ -59,13 +59,18 @@ func TestStateBagToTag_CreateFilter(t *testing.T) {
 			args: []interface{}{""},
 			err:  filters.ErrInvalidFilterParameters,
 		},
+		{
+			msg:  "too many args",
+			args: []interface{}{"foo", "bar", "baz"},
+			err:  filters.ErrInvalidFilterParameters,
+		},
 	} {
 		t.Run(ti.msg, func(t *testing.T) {
 			f, err := NewStateBagToTag().CreateFilter(ti.args)
 
 			assert.Equal(t, ti.err, err)
 			if err == nil {
-				ff := f.(stateBagToTagFilter)
+				ff := f.(*stateBagToTagFilter)
 
 				assert.Equal(t, ti.stateBag, ff.stateBagItemName)
 				assert.Equal(t, ti.tag, ff.tagName)