[metrics] Introduce metrics API (#39202)

[metrics] Introduce metrics API (#39202)

See the [design
doc](https://github.com/systemd/systemd/issues/38023#issue-3192567450)
by @ikruglov

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

### 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.

### 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

### 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
}
```