Use different monitoring bulk API paths depending on ES version

[ycombinator]

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:

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

[elasticmachine]

Pinging @elastic/stack-monitoring

[ycombinator]

Blocked by #11204.

[jakelandis]

The paths look correct, and that probably as much I am competent to review here, so (lowercase) lgtm

[urso]

I wish we would not have to pass the versions to MonitoringBulkWith. Actually (*Connection) stores the versions, but this means that one has to call Connect before using this method.

[ycombinator]

@urso So originally (see 96a8eba) I had it so the version wasn't being passed into MonitoringBulkWith. WDYT of that approach?

[urso]

more like:

func (c *Connection) SendMonitoring(params map[string]string, body []interface{}) error {
    ...

    if !c.version.IsValid() {
      if err := c.Connect(); err != nil {
        return err
      }
    }

    ...
    requ, err := newMonitoringBulkRequest(c.version, conn.URL, params, enc)
	if err != nil {
		return nil, err
	}
   
    ...
} 

[ycombinator]

@urso I made the change per your review suggestion. This is ready for your 👀 again. Thanks!

[ph]

I've discussed with @urso last week and I think we will merge this one as is, we need a better way to implement different version calls in our clients.