Add tests to verify template content

We recently started to move fields.yml into the Golang binary to be used internally. To make sure the loading important and loading of all the data into the binary works as expected for Metricbeat, this adds some basic tests. Related to elastic#7605.

metricbeat/tests/system/test_template.py

@@ -0,0 +1,55 @@
+import os
+import metricbeat
+import json
+from nose.plugins.skip import SkipTest
+
+
+class Test(metricbeat.BaseTest):
+
+    def test_export_template(self):
+        """
+        Test export template works and contains all fields
+        """
+
+        if os.name == "nt":
+            raise SkipTest
+
+        self.render_config_template("metricbeat",
+                                    os.path.join(self.working_dir,
+                                                 "metricbeat.yml"),
+                                    )
+
+        # Remove fields.yml to make sure template is built from internal binary data
+        os.remove(os.path.join(self.working_dir, "fields.yml"))
+
+        exit_code = self.run_beat(
+            logging_args=[],
+            extra_args=["export", "template"],
+            config="metricbeat.yml",
+            output="template.json"
+        )
+        assert exit_code == 0
+
+        template_path = os.path.join(self.working_dir, "template.json")
+        template_content = ""
+
+        # Read in all json lines and discard the coverage info
+        with open(template_path) as f:
+            for line in f:
+                template_content += line
+                if line.startswith("}"):
+                    break
+
+        t = json.loads(template_content)
+
+        properties = t["mappings"]["doc"]["properties"]
+
+        # Check libbeat fields
+        assert properties["@timestamp"] == {"type": "date"}
+        assert properties["host"]["properties"]["name"] == {"type": "keyword", "ignore_above": 1024}
+
+        # Check metricbeat generic field
+        assert properties["metricset"]["properties"]["host"] == {"type": "keyword", "ignore_above": 1024}
+
+        # Check module specific field
+        assert properties["system"]["properties"]["cpu"]["properties"]["cores"] == {"type": "long"}