[metrics] Introduce metrics API

[YapingLi04]

See the design doc by @ikruglov

This PR introduces the metrics API framework, adds some basic system wide and per unit metrics, and a basic CLI. The PR is broken into two commits as described below.

First commit

The first commit includes:

• Metrics API definitions
• Code to set up the varlink server
• The describe method which shows all the metrics families
• The list method which lists all the metrics
• Type definitions related to MetricFamily
• Common code to build json objects

Second commit

The second commit adds some basic metrics, a basic CLI (systemd-report) which
lists the metrics, and integration tests.

System wide metrics:

• units_by_type_total
• units_by_state_total

Two per unit metrics:

• unit_active_state
• unit_load_state

A service state metric:

• nrestarts

Deviations from the original design

• Introduced top level field "object" for ease of filtering. Instead of having fields: { unit: "foo", unit_type: "service" }, we now have object: foo.service as the top level field.

Sample outputs

units_by_type_total:

{
        "name" : "io.systemd.Manager.units_by_type_total",
        "value" : 52,
        "fields" : {
                "type" : "target"
        }
}
{
        "name" : "io.systemd.Manager.units_by_type_total",
        "value" : 82,
        "fields" : {
                "type" : "device"
        }
}
{
        "name" : "io.systemd.Manager.units_by_type_total",
        "value" : 2,
        "fields" : {
                "type" : "automount"
        }
}

units_by_state_total:

{
        "name" : "io.systemd.Manager.units_by_state_total",
        "value" : 216,
        "fields" : {
                "state" : "active"
        }
}
{
        "name" : "io.systemd.Manager.units_by_state_total",
        "value" : 0,
        "fields" : {
                "state" : "reloading"
        }
}
{
        "name" : "io.systemd.Manager.units_by_state_total",
        "value" : 120,
        "fields" : {
                "state" : "inactive"
        }
}

unit_active_state:

{
        "name" : "io.systemd.Manager.unit_active_state",
        "object" : "multi-user.target",
        "value" : "active"
}
{
        "name" : "io.systemd.Manager.unit_active_state",
        "object" : "systemd-sysusers.service",
        "value" : "inactive"
}

unit_load_state:

{
        "name" : "io.systemd.Manager.unit_load_state",
        "object" : "multi-user.target",
        "value" : "loaded"
}

nrestarts:

{
        "name" : "io.systemd.Manager.nrestarts",
        "object" : "user@0.service",
        "value" : 0
}
{
        "name" : "io.systemd.Manager.nrestarts",
        "object" : "user-runtime-dir@0.service",
        "value" : 0
}