update IsServerless() to use correct Kibana APIs

[fearful-symmetry]

What does this PR do?

closes #181

The initial implementation of Client.IsServerless() was implemented as a hack before we had any kind of API; this updates the method to use the /api/status endpoint that now reports the build type.

You can test this against a "real" Kibana instance like this:

func TestServerlessInt(t *testing.T) {
	conn := Connection{
		URL:    "KIBANA_URL",
		HTTP:   http.DefaultClient,
		APIKey: "BEATS_API_KEY",
	}

	testClient := Client{
		Connection: conn,
	}

	got, err := testClient.KibanaIsServerless()
	require.NoError(t, err)
	require.True(t, got)

}

Why is it important?

The current implementation is broken.

Checklist

• My code follows the style guidelines of this project
• I have commented my code, particularly in hard-to-understand areas
• I have added tests that prove my fix is effective or that my feature works

[elasticmachine]

💚 Build Succeeded

• Buildkite Build
• Commit: 290b1ca

History

• 💔 Build #333 failed f906784
• 💔 Build #332 failed 85ec709

cc @fearful-symmetry

[TinaHeiligers]

Thanks for handling this @fearful-symmetry !