Don't generate autodiscover config when no port matches host hints (elastic#12086)

On metricbeat, when the host autodiscover hint is used, and it includes
the port, one of the exposed ports has to match with the one in the
hint. If not, no configuration should be generated. If it is generated,
it will have empty hosts, what would lead to unexpected errors as the
seen in elastic#8264.

(cherry picked from commit cc73643)

Author: jsoriano

CHANGELOG.next.asciidoc

@@ -43,6 +43,7 @@ https://github.com/elastic/beats/compare/v7.0.0...7.1[Check the HEAD diff]
 *Metricbeat*
 
 - Update documentation with cloudwatch:ListMetrics permission. {pull}11987[11987]
+- Avoid generating hints-based configuration with empty hosts when no exposed port is suitable for the hosts hint. {issue}8264[8264] {pull}12086[12086]
 
 *Packetbeat*
 

metricbeat/autodiscover/builder/hints/metrics.go

@@ -101,7 +101,11 @@ func (m *metricHints) CreateConfig(event bus.Event) []*common.Config {
 		return config
 	}
 
-	hsts := m.getHostsWithPort(hints, port)
+	hosts, ok := m.getHostsWithPort(hints, port)
+	if !ok {
+		return config
+	}
+
 	ns := m.getNamespace(hints)
 	msets := m.getMetricSets(hints, mod)
 	tout := m.getTimeout(hints)
@@ -112,7 +116,7 @@ func (m *metricHints) CreateConfig(event bus.Event) []*common.Config {
 	moduleConfig := common.MapStr{
 		"module":     mod,
 		"metricsets": msets,
-		"hosts":      hsts,
+		"hosts":      hosts,
 		"timeout":    tout,
 		"period":     ival,
 		"enabled":    true,
@@ -161,7 +165,7 @@ func (m *metricHints) getMetricSets(hints common.MapStr, module string) []string
 	return msets
 }
 
-func (m *metricHints) getHostsWithPort(hints common.MapStr, port int) []string {
+func (m *metricHints) getHostsWithPort(hints common.MapStr, port int) ([]string, bool) {
 	var result []string
 	thosts := builder.GetHintAsList(hints, m.Key, hosts)
 
@@ -175,7 +179,12 @@ func (m *metricHints) getHostsWithPort(hints common.MapStr, port int) []string {
 		}
 	}
 
-	return result
+	if len(thosts) > 0 && len(result) == 0 {
+		logp.Debug("hints.builder", "no hosts selected for port %d with hints: %+v", port, thosts)
+		return nil, false
+	}
+
+	return result, true
 }
 
 func (m *metricHints) getNamespace(hints common.MapStr) string {

metricbeat/autodiscover/builder/hints/metrics_test.go

@@ -67,6 +67,65 @@ func TestGenerateHints(t *testing.T) {
 			len:    0,
 			result: common.MapStr{},
 		},
+		{
+			message: "Hints without matching port should return nothing",
+			event: bus.Event{
+				"host": "1.2.3.4",
+				"port": 9090,
+				"hints": common.MapStr{
+					"metrics": common.MapStr{
+						"module": "mockmoduledefaults",
+						"hosts":  "${data.host}:8888",
+					},
+				},
+			},
+			len:    0,
+			result: common.MapStr{},
+		},
+		{
+			message: "Hints with multiple hosts return only the matching one",
+			event: bus.Event{
+				"host": "1.2.3.4",
+				"port": 9090,
+				"hints": common.MapStr{
+					"metrics": common.MapStr{
+						"module": "mockmoduledefaults",
+						"hosts":  "${data.host}:8888,${data.host}:9090",
+					},
+				},
+			},
+			len: 1,
+			result: common.MapStr{
+				"module":     "mockmoduledefaults",
+				"metricsets": []string{"default"},
+				"timeout":    "3s",
+				"period":     "1m",
+				"enabled":    true,
+				"hosts":      []interface{}{"1.2.3.4:9090"},
+			},
+		},
+		{
+			message: "Hints with multiple hosts return only the one with the template",
+			event: bus.Event{
+				"host": "1.2.3.4",
+				"port": 9090,
+				"hints": common.MapStr{
+					"metrics": common.MapStr{
+						"module": "mockmoduledefaults",
+						"hosts":  "${data.host}:8888,${data.host}:${data.port}",
+					},
+				},
+			},
+			len: 1,
+			result: common.MapStr{
+				"module":     "mockmoduledefaults",
+				"metricsets": []string{"default"},
+				"timeout":    "3s",
+				"period":     "1m",
+				"enabled":    true,
+				"hosts":      []interface{}{"1.2.3.4:9090"},
+			},
+		},
 		{
 			message: "Only module hint should return all metricsets",
 			event: bus.Event{
@@ -87,7 +146,7 @@ func TestGenerateHints(t *testing.T) {
 			},
 		},
 		{
-			message: "metricsets hint works",
+			message: "Metricsets hint works",
 			event: bus.Event{
 				"host": "1.2.3.4",
 				"hints": common.MapStr{