func AttachmentsFromContext(ctx context.Context) Attachments {
	var a Attachments
	for _, extractor := range extractors {
		a = extractor(ctx, a)
	}
	return a
}
NewFromContext extracts exemplars from the given context.
Each registered AttachmentExtractor (see RegisterAttachmentExtractor) is called in an
unspecified order to add attachments to the exemplar.
package exemplar
import (
	"context"
	"time"
)
const (
	KeyTraceID   = "trace_id"
	KeySpanID    = "span_id"
	KeyPrefixTag = "tag:"
)
type Exemplar struct {
	Value       float64      
	Timestamp   time.Time    
	Attachments Attachments  
}
type Attachments map[string]string
type AttachmentExtractor func(ctx context.Context, a Attachments) Attachments
var extractors []AttachmentExtractor
func RegisterAttachmentExtractor(e AttachmentExtractor) {
	extractors = append(extractors, e)
}