Improve systemd configuration and metric names (#3974)

upgrade:
- Update to the configuration of the systemd check: ``unit_names`` is now required and only matching units will be monitored, ``unit_regexes`` configuration has been removed.
- Several metrics sent by the systemd check have been renamed. The integration is now stable.

enhancements:
- Add `private_socket` configuration to the systemd check. Defaults to `/run/systemd/private` (or `/host/run/systemd/private` when using Docker Agent).

NOTICE

@@ -2,3 +2,4 @@ Datadog datadog-agent
 Copyright 2016-2019 Datadog, Inc.
 
 This product includes software developed at Datadog (https://www.datadoghq.com/).
+This product includes software developed at CoreOS, Inc (http://www.coreos.com/).

cmd/agent/dist/conf.d/systemd.d/conf.yaml.example

@@ -1,27 +1,23 @@
 init_config:
 
 instances:
-    -
-    ## @param unit_names - list of strings - optional
-    ## List of systemd units to monitor (full name e.g. ssh.service).
-    ## If `unit_names` and `unit_regexes` are both NOT specified, the check monitors all units.
+    ## @param unit_names - list of strings - required
+    ## List of systemd units to monitor.
+    ## Full names must be used. Examples: ssh.service, docker.socket
     #
-    # unit_names:
-    #   - <UNIT_NAME>
+  - unit_names:
+      - <UNIT_NAME>
 
-    ## @param unit_regexes - list of strings - optional
-    ## Regex pattern[s] matching the names of units to monitor.
-    ## If `unit_names` and `unit_regexes` are both NOT specified, the check monitors all units.
-    ## 
-    ## If the number of units managed by systemd is unbounded, it might increase
-    ## the number of custom metrics send by this check which may impact your billing.
-    ## Learn more about custom metrics: https://docs.datadoghq.com/developers/metrics/custom_metrics/#how-is-a-custom-metric-defined
-    #
-    # unit_regexes:
-    #   - ^<UNIT_NAME>$        # match exactly the unit name "<UNIT_NAME>"
+    ## @param private_socket - string - optional
+    ## Path to systemd private socket needed to retrieve systemd data.
+    ## Defaults to `/run/systemd/private` or `/host/run/systemd/private` when
+    ## using Docker Agent (https://docs.datadoghq.com/agent/docker/).
+    ##
+    # private_socket: <PATH_TO_SYSTEMD_PRIVATE_SOCKET>
 
     ## @param tags  - list of key:value elements - optional
-    ## List of tags to attach to every metric, event, and service check emitted by this integration.
+    ## List of tags to attach to every metric, event, and service check emitted
+    ## by this integration.
     ##
     ## Learn more about tagging at https://docs.datadoghq.com/tagging
     #

pkg/collector/corechecks/systemd/dbus_conn.go

@@ -0,0 +1,59 @@
+// Unless explicitly stated otherwise all files in this repository are licensed
+// under the Apache License Version 2.0.
+// This product includes software developed at Datadog (https://www.datadoghq.com/).
+// This file includes software developed at CoreOS, Inc (http://www.coreos.com/).
+
+// Copyright 2016-2019 Datadog, Inc.
+// Copyright 2015 CoreOS, Inc.
+//
+// Use of this source code is governed by Apache License 2.0
+// license that can be found here: https://github.com/coreos/go-systemd/blob/master/LICENSE
+
+// +build systemd
+
+package systemd
+
+import (
+	"fmt"
+	"os"
+	"strconv"
+
+	"github.com/coreos/go-systemd/dbus"
+	godbus "github.com/godbus/dbus"
+)
+
+// NewSystemdConnection establishes a private, direct connection to systemd.
+// This can be used for communicating with systemd without a dbus daemon.
+// Callers should call Close() when done with the connection.
+// Note: method borrowed from `go-systemd/dbus` to provide custom path for systemd private socket
+// Source: https://github.com/coreos/go-systemd/blob/master/dbus/dbus.go
+func NewSystemdConnection(privateSocket string) (*dbus.Conn, error) {
+	return dbus.NewConnection(func() (*godbus.Conn, error) {
+		// We skip Hello when talking directly to systemd.
+		return dbusAuthConnection(func() (*godbus.Conn, error) {
+			return godbus.Dial(fmt.Sprintf("unix:path=%s", privateSocket))
+		})
+	})
+}
+
+// Note: method borrowed from `go-systemd/dbus` to provide custom path for systemd private socket
+// Source: https://github.com/coreos/go-systemd/blob/master/dbus/dbus.go
+func dbusAuthConnection(createBus func() (*godbus.Conn, error)) (*godbus.Conn, error) {
+	conn, err := createBus()
+	if err != nil {
+		return nil, err
+	}
+
+	// Only use EXTERNAL method, and hardcode the uid (not username)
+	// to avoid a username lookup (which requires a dynamically linked
+	// libc)
+	methods := []godbus.Auth{godbus.AuthExternal(strconv.Itoa(os.Getuid()))}
+
+	err = conn.Auth(methods)
+	if err != nil {
+		conn.Close()
+		return nil, err
+	}
+
+	return conn, nil
+}