Getting avro message metadata

Nobody has claimed this yet.

In my system I need to get `Schema` from Avro message, and I notice we could get `Schema` by calling `(*OCFReader).Metadata()`. But when generating new `OCFReader` it has to generate new `codec`, and I see this is quite expensive to do. Ref: https://github.com/linkedin/goavro/blob/master/ocf.go#L114-L171

So, I propose to create function that will return Avro metadata when called, without creating new `OCFReader`. This is what I used now:
```
func readAvroHeader(ctx context.Context, ior io.Reader) (map[string][]byte, error) {
span, ctx := tracer.StartSpanFromContext(ctx)
defer span.Finish()

//
// magic bytes
//
magic := make([]byte, 4)
_, err := io.ReadFull(ior, magic)
if err != nil {
return nil, fmt.Errorf("cannot read OCF header magic bytes: %s", err)
}

if !bytes.Equal(magic, ocfMagicBytes) {
return nil, fmt.Errorf("cannot read OCF header with invalid magic bytes: %#q", magic)
}

//
// metadata
//
metadata, err := metadataBinaryReader(ctx, ior)
if err != nil {
return nil, fmt.Errorf("cannot read OCF header metadata: %s", err)
}
return metadata, nil
}
```

What do you guys think?

Review ocf.go around the OCFReader constructor and Metadata method, then inspect metadataBinaryReader and the proposed header-reading flow. Define an accessible metadata path that avoids constructing a new codec or OCFReader, and verify that it reads the OCF magic bytes and metadata with the existing error behavior.

Written by the indexing model from the issue text.