Add connection metricset of nats module (#10095)

This PR adds `connections` metricset to `nats` module as part of #10071. 

`connections` metricset retrieves metrics from `connz` [monitoring](https://nats.io/documentation/tutorials/nats-monitoring/) URI of `nats`.   


_______________________________________________________________________________

Co-Authored-By: Stamatis Katsaounis <katsaouniss@gmail.com>, @skatsaounis 
Co-Authored-By: Michael Katsoulis <michaelkatsoulis88@gmail.com>, @MichaelKatsoulis

CHANGELOG.asciidoc

@@ -66,7 +66,6 @@ https://github.com/elastic/beats/compare/v7.0.0-alpha2...master[Check the HEAD d
 *Journalbeat*
 
 *Metricbeat*
-- Add nats module with stats metricset. {pull}9825[9825]
 
 *Packetbeat*
 
@@ -102,6 +101,8 @@ https://github.com/elastic/beats/compare/v7.0.0-alpha2...master[Check the HEAD d
 
 - Add `key` metricset to the Redis module. {issue}9582[9582] {pull}9657[9657] {pull}9746[9746]
 - Add `socket_summary` metricset to system defaults, removing experimental tag and supporting Windows {pull}9709[9709]
+- Add `nats` module with `stats` metricset. {pull}9825[9825]
+- Add `connections` metricset to the `nats` module. {pull}10095[10095]
 
 *Packetbeat*
 

metricbeat/docs/fields.asciidoc

@@ -17232,33 +17232,50 @@ nats Module
 
 
 
-[float]
-== stats fields
+*`nats.server_id`*::
++
+--
+type: string
 
-Contains nats var related metrics
+The server ID
 
 
+--
 
-*`nats.stats.server_id`*::
+*`nats.now`*::
 +
 --
-type: keyword
+type: date
 
-The server ID
+Server time of metric creation
 
 
 --
 
-*`nats.stats.now`*::
+[float]
+== connections fields
+
+Contains nats connection related metrics
+
+
+
+*`nats.connections.total`*::
 +
 --
-type: date
+type: integer
 
-Server time of metric creation
+The number of currently active clients
 
 
 --
 
+[float]
+== stats fields
+
+Contains nats var related metrics
+
+
+
 *`nats.stats.uptime`*::
 +
 --

metricbeat/docs/modules/nats.asciidoc

@@ -29,18 +29,23 @@ in <<configuration-metricbeat>>. Here is an example configuration:
 ----
 metricbeat.modules:
 - module: nats
-  metricsets: ["stats"]
+  metricsets: ["connections", "stats"]
   period: 10s
   hosts: ["localhost:8222"]
   #stats.metrics_path: "/varz"
+  #connections.metrics_path: "/connz"
 ----
 
 [float]
 === Metricsets
 
 The following metricsets are available:
 
+* <<metricbeat-metricset-nats-connections,connections>>
+
 * <<metricbeat-metricset-nats-stats,stats>>
 
+include::nats/connections.asciidoc[]
+
 include::nats/stats.asciidoc[]
 

metricbeat/docs/modules/nats/connections.asciidoc

@@ -0,0 +1,23 @@
+////
+This file is generated! See scripts/docs_collector.py
+////
+
+[[metricbeat-metricset-nats-connections]]
+=== Nats connections metricset
+
+experimental[]
+
+include::../../../module/nats/connections/_meta/docs.asciidoc[]
+
+
+==== Fields
+
+For a description of each field in the metricset, see the
+<<exported-fields-nats,exported fields>> section.
+
+Here is an example document generated by this metricset:
+
+[source,json]
+----
+include::../../../module/nats/connections/_meta/data.json[]
+----

metricbeat/docs/modules_list.asciidoc

@@ -102,7 +102,8 @@ This file is generated! See scripts/docs_collector.py
 .2+| .2+|  |<<metricbeat-metricset-mysql-galera_status,galera_status>> experimental[]  
 |<<metricbeat-metricset-mysql-status,status>>   
 |<<metricbeat-module-nats,Nats>>  experimental[]   |image:./images/icon-no.png[No prebuilt dashboards]    |  
-.1+| .1+|  |<<metricbeat-metricset-nats-stats,stats>> experimental[]  
+.2+| .2+|  |<<metricbeat-metricset-nats-connections,connections>> experimental[]  
+|<<metricbeat-metricset-nats-stats,stats>> experimental[]  
 |<<metricbeat-module-nginx,Nginx>>     |image:./images/icon-yes.png[Prebuilt dashboards are available]    |  
 .1+| .1+|  |<<metricbeat-metricset-nginx-stubstatus,stubstatus>>   
 |<<metricbeat-module-php_fpm,PHP_FPM>>  beta[]   |image:./images/icon-no.png[No prebuilt dashboards]    |  

metricbeat/metricbeat.reference.yml

@@ -518,10 +518,11 @@ metricbeat.modules:
 
 #-------------------------------- Nats Module --------------------------------
 - module: nats
-  metricsets: ["stats"]
+  metricsets: ["connections", "stats"]
   period: 10s
   hosts: ["localhost:8222"]
   #stats.metrics_path: "/varz"
+  #connections.metrics_path: "/connz"
 
 #-------------------------------- Nginx Module -------------------------------
 - module: nginx

metricbeat/module/nats/_meta/config.reference.yml

@@ -1,5 +1,6 @@
 - module: nats
-  metricsets: ["stats"]
+  metricsets: ["connections", "stats"]
   period: 10s
   hosts: ["localhost:8222"]
   #stats.metrics_path: "/varz"
+  #connections.metrics_path: "/connz"

metricbeat/module/nats/_meta/config.yml

@@ -1,5 +1,6 @@
 - module: nats
-  metricsets: ["stats"]
+  metricsets: ["connections", "stats"]
   period: 10s
   hosts: ["localhost:8222"]
-  stats.metrics_path: "/varz"
+  #stats.metrics_path: "/varz"
+  #connections.metrics_path: "/connz"

metricbeat/module/nats/_meta/fields.yml

@@ -8,3 +8,11 @@
       description: >
         `nats` contains statistics that were read from Nats
       fields:
+        - name: server_id
+          type: string
+          description: >
+            The server ID
+        - name: now
+          type: date
+          description: >
+            Server time of metric creation

metricbeat/module/nats/connections/_meta/data.json

@@ -0,0 +1,21 @@
+{
+    "@timestamp":"2016-05-23T08:05:34.853Z",
+    "beat":{
+        "hostname":"beathost",
+        "name":"beathost"
+    },
+    "metricset":{
+        "host":"localhost",
+        "module":"nats",
+        "name":"connections",
+        "rtt":44269
+    },
+    "nats":{
+        "server_id": "bUAdpRFtMWddIBWw80Yd9D",
+        "now": "2018-12-28T12:33:53.026865597Z",
+        "connections":{
+            "total": 10
+        }
+    },
+    "type":"metricsets"
+}

metricbeat/module/nats/connections/_meta/docs.asciidoc

@@ -0,0 +1 @@
+This is the connections metricset of the module nats.

metricbeat/module/nats/connections/_meta/fields.yml

@@ -0,0 +1,9 @@
+- name: connections
+  type: group
+  description: >
+    Contains nats connection related metrics
+  fields:
+    - name: total
+      type: integer
+      description: >
+        The number of currently active clients

metricbeat/module/nats/connections/_meta/test/connectionsmetrics.json

@@ -0,0 +1,5 @@
+{
+    "server_id": "bUAdpRFtMWddIBWw80Yd9D",
+    "now": "2018-12-28T12:33:53.026865597Z",
+    "total": 10
+}

metricbeat/module/nats/connections/connections.go

@@ -0,0 +1,98 @@
+// Licensed to Elasticsearch B.V. under one or more contributor
+// license agreements. See the NOTICE file distributed with
+// this work for additional information regarding copyright
+// ownership. Elasticsearch B.V. licenses this file to you under
+// the Apache License, Version 2.0 (the "License"); you may
+// not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing,
+// software distributed under the License is distributed on an
+// "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+// KIND, either express or implied.  See the License for the
+// specific language governing permissions and limitations
+// under the License.
+
+package connections
+
+import (
+	"github.com/elastic/beats/libbeat/common/cfgwarn"
+	"github.com/elastic/beats/libbeat/logp"
+	"github.com/elastic/beats/metricbeat/helper"
+	"github.com/elastic/beats/metricbeat/helper/elastic"
+	"github.com/elastic/beats/metricbeat/mb"
+	"github.com/elastic/beats/metricbeat/mb/parse"
+)
+
+const (
+	defaultScheme = "http"
+	defaultPath   = "/connz"
+)
+
+var (
+	hostParser = parse.URLHostParserBuilder{
+		DefaultScheme: defaultScheme,
+		DefaultPath:   defaultPath,
+		PathConfigKey: "connections.metrics_path",
+	}.Build()
+)
+
+// init registers the MetricSet with the central registry as soon as the program
+// starts. The New function will be called later to instantiate an instance of
+// the MetricSet for each host defined in the module's configuration. After the
+// MetricSet has been created then Fetch will begin to be called periodically.
+func init() {
+	mb.Registry.MustAddMetricSet("nats", "connections", New,
+		mb.WithHostParser(hostParser),
+		mb.DefaultMetricSet(),
+	)
+}
+
+// MetricSet holds any configuration or state information. It must implement
+// the mb.MetricSet interface. And this is best achieved by embedding
+// mb.BaseMetricSet because it implements all of the required mb.MetricSet
+// interface methods except for Fetch.
+type MetricSet struct {
+	mb.BaseMetricSet
+	http *helper.HTTP
+	Log  *logp.Logger
+}
+
+// New creates a new instance of the MetricSet. New is responsible for unpacking
+// any MetricSet specific configuration options if there are any.
+func New(base mb.BaseMetricSet) (mb.MetricSet, error) {
+	cfgwarn.Experimental("The nats connections metricset is experimental.")
+
+	config := struct{}{}
+	if err := base.Module().UnpackConfig(&config); err != nil {
+		return nil, err
+	}
+
+	http, err := helper.NewHTTP(base)
+	if err != nil {
+		return nil, err
+	}
+	return &MetricSet{
+		base,
+		http,
+		logp.NewLogger("nats"),
+	}, nil
+}
+
+// Fetch methods implements the data gathering and data conversion to the right
+// format. It publishes the event which is then forwarded to the output. In case
+// of an error set the Error field of mb.Event or simply call report.Error().
+func (m *MetricSet) Fetch(r mb.ReporterV2) {
+	content, err := m.http.FetchContent()
+	if err != nil {
+		elastic.ReportAndLogError(err, r, m.Log)
+		return
+	}
+	err = eventMapping(r, content)
+	if err != nil {
+		elastic.ReportAndLogError(err, r, m.Log)
+		return
+	}
+}