func (s *FieldCapsService) Index(index ...string) *FieldCapsService {
	s.index = append(s.index, index...)
	return s
}
Index is a list of index names; use `_all` or empty string to perform
the operation on all indices.
package elastic
import (
	"context"
	"fmt"
	"net/http"
	"net/url"
	"strings"
	"github.com/olivere/elastic/uritemplates"
)
type FieldCapsService struct {
	client            *Client
	pretty            bool
	index             []string
	allowNoIndices    *bool
	expandWildcards   string
	fields            []string
	ignoreUnavailable *bool
	bodyJson          interface{}
	bodyString        string
}
func NewFieldCapsService(client *Client) *FieldCapsService {
	return &FieldCapsService{
		client: client,
	}
}