Cherry-pick #11203 to 7.0: Use different monitoring bulk API paths de…

…pending on ES version (#11323)

Cherry-pick of PR #11203 to 7.0 branch. Original message: 

Resolves #9480.

Starting Elasticsearch 7.0.0, Beats should ship their monitoring data to the `_monitoring/bulk` Elasticsearch API endpoint. Prior to 7.0.0, `_xpack/monitoring/_bulk` should be used. This PR implements this version-based conditional logic.

I used Wireshark to look at the ES API endpoints being hit.

Running this PR with ES 8.0.0 or ES 7.0.0, I confirmed that the `POST _monitoring/bulk` endpoint was being hit:

<img width="1436" alt="Screen Shot 2019-03-14 at 10 55 52 AM" src="https://user-images.githubusercontent.com/51061/54380101-ed567780-4647-11e9-8ed1-9b9020bb85d4.png">

And running this PR with ES 6.7.0, I confirmed that the `POST _xpack/monitoring/_bulk` endpoint was being hit:

<img width="1437" alt="Screen Shot 2019-03-14 at 10 56 42 AM" src="https://user-images.githubusercontent.com/51061/54380094-eaf41d80-4647-11e9-8658-d9a6ba14541b.png">

libbeat/monitoring/report/elasticsearch/client.go

@@ -128,7 +128,8 @@ func (c *publishClient) Publish(batch publisher.Batch) error {
 
 		// Currently one request per event is sent. Reason is that each event can contain different
 		// interval params and X-Pack requires to send the interval param.
-		_, err = c.es.BulkWith("_xpack", "monitoring", params, nil, bulk[:])
+		_, err = c.es.SendMonitoringBulk(params, bulk[:])
+
 		if err != nil {
 			failed = append(failed, event)
 			reason = err

libbeat/outputs/elasticsearch/bulkapi.go

@@ -23,6 +23,8 @@ import (
 	"io/ioutil"
 	"net/http"
 	"strings"
+
+	"github.com/elastic/beats/libbeat/common"
 )
 
 // MetaBuilder creates meta data for bulk requests
@@ -77,6 +79,41 @@ func (conn *Connection) BulkWith(
 	return readQueryResult(result.raw)
 }
 
+// SendMonitoringBulk creates a HTTP request to the X-Pack Monitoring API containing a bunch of
+// operations and sends them to Elasticsearch. The request is retransmitted up to max_retries
+// before returning an error.
+func (conn *Connection) SendMonitoringBulk(
+	params map[string]string,
+	body []interface{},
+) (*QueryResult, error) {
+	if len(body) == 0 {
+		return nil, nil
+	}
+
+	enc := conn.encoder
+	enc.Reset()
+	if err := bulkEncode(enc, nil, body); err != nil {
+		return nil, err
+	}
+
+	if !conn.version.IsValid() {
+		if err := conn.Connect(); err != nil {
+			return nil, err
+		}
+	}
+
+	requ, err := newMonitoringBulkRequest(conn.version, conn.URL, params, enc)
+	if err != nil {
+		return nil, err
+	}
+
+	_, result, err := conn.sendBulkRequest(requ)
+	if err != nil {
+		return nil, err
+	}
+	return readQueryResult(result.raw)
+}
+
 func newBulkRequest(
 	urlStr string,
 	index, docType string,
@@ -88,6 +125,36 @@ func newBulkRequest(
 		return nil, err
 	}
 
+	return newBulkRequestWithPath(urlStr, path, params, body)
+}
+
+func newMonitoringBulkRequest(
+	esVersion common.Version,
+	urlStr string,
+	params map[string]string,
+	body bodyEncoder,
+) (*bulkRequest, error) {
+	var path string
+	var err error
+	if esVersion.Major < 7 {
+		path, err = makePath("_xpack", "monitoring", "_bulk")
+	} else {
+		path, err = makePath("_monitoring", "bulk", "")
+	}
+
+	if err != nil {
+		return nil, err
+	}
+
+	return newBulkRequestWithPath(urlStr, path, params, body)
+}
+
+func newBulkRequestWithPath(
+	urlStr string,
+	path string,
+	params map[string]string,
+	body bodyEncoder,
+) (*bulkRequest, error) {
 	url := addToURL(urlStr, path, "", params)
 
 	var reader io.Reader