[metrics] Add basic system wide and per unit metrics

[metrics] Add basic system wide and per unit metrics

This commit adds some basic metrics and integration tests.

System wide metrics:
- units_by_type_total: target/device/automount etc.
- units_by_state_total: active/reloading/inactive etc.

Two per unit metrics which shows the current state of a unit:
- unit_active_state
- unit_load_state

A metric for service state:
- nrestarts

Here are some 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
}