[Heartbeat] Fix exit on disabled monitor

Fixes a bug where when `enabled: false` was set on a monitor heartbeat
would refuse to start.

Fixes elastic#22665

heartbeat/beater/heartbeat.go

@@ -21,6 +21,8 @@ import (
 	"fmt"
 	"time"
 
+	"github.com/elastic/beats/v7/heartbeat/monitors/stdfields"
+
 	"github.com/elastic/beats/v7/heartbeat/hbregistry"
 
 	"github.com/pkg/errors"
@@ -127,8 +129,13 @@ func (bt *Heartbeat) RunStaticMonitors(b *beat.Beat) error {
 	for _, cfg := range bt.config.Monitors {
 		created, err := factory.Create(b.Publisher, cfg)
 		if err != nil {
+			if err == stdfields.ErrPluginDisabled {
+				continue // don't stop loading monitors just because they're disabled
+			}
+
 			return errors.Wrap(err, "could not create monitor")
 		}
+
 		created.Start()
 	}
 	return nil

heartbeat/monitors/stdfields/stdfields.go

@@ -28,7 +28,7 @@ import (
 )
 
 // ErrPluginDisabled is returned when the monitor plugin is marked as disabled.
-var ErrPluginDisabled = errors.New("Monitor not loaded, plugin is disabled")
+var ErrPluginDisabled = errors.New("monitor not loaded, plugin is disabled")
 
 type ServiceFields struct {
 	Name string `config:"name"`

heartbeat/tests/system/test_base.py

@@ -32,6 +32,30 @@ def test_base(self):
         self.wait_until(lambda: self.log_contains("heartbeat is running"))
         heartbeat_proc.check_kill_and_wait()
 
+    def test_disabled(self):
+        """
+        Basic test against a disabled monitor
+        """
+
+        config = {
+            "monitors": [
+                {
+                    "type": "http",
+                    "enabled": "false",
+                    "urls": ["http://localhost:9200"],
+                }
+            ]
+        }
+
+        self.render_config_template(
+            path=os.path.abspath(self.working_dir) + "/log/*",
+            **config
+        )
+
+        heartbeat_proc = self.start_beat()
+        self.wait_until(lambda: self.log_contains("heartbeat is running"))
+        heartbeat_proc.check_kill_and_wait()
+
     def test_fields_under_root(self):
         """
         Basic test with fields and tags in monitor