func getHeaderValue(headers map[string]string, key string) string {
	if value, ok := headers[key]; ok {
		return value
	}
	return ""
}
getHeaderValue is a helper function that is guaranteed to return a string. Given a key, it
attempts to find the associated value in the provided header. If none is found, it returns
an empty string.
package propagation
import (
	"fmt"
)