Why is metadataAnnotators not []func(metadata.MD, *http.Request)

I don't think 
```go
for _, mda := range mux.metadataAnnotators {
		md = metadata.Join(md, mda(ctx, req))
	}
```
 is better than 
```go
for _, mda := range mux.metadataAnnotators {
		mda(md, req)
	}
```