Resolves: epoch8#54, epoch8#55, epoch8#56

More info:

This commit adds the support for the configuration of the exporter via
`prometheus_exporter` section of Airflow configuration.

The keys with `expose_` prefix enable or disable the exposure of certain class
of metrics. For example, Airflow variables-related metrics are not exposed by
default. However, with `expose_variables` set to `True`, the exporter will start
exposing the metrics. Similarly, by default, the exporter exposes scheduler-related
metrics. However, with `expose_scheduler` set to `False`, the exporter will
not export them.

Additionally, when `expose_config` is being enabled, the exporter will expose
a subset of `core` and `prometheus_exporter` configuration settings.

```
[prometheus_exporter]
auth_enabled = True
auth_token = 4e1dba1c-2b66-4275-b8ae-292ee9665fa1
expose_variables = True
expose_config = False
expose_scheduler = False
```

It is possible to disable the exporter:

```
[prometheus_exporter]
disabled = True
```

When authentication is enabled, the metrics are accessible via:

```bash
curl -v -H 'Authorization: Bearer 4e1dba1c-2b66-4275-b8ae-292ee9665fa1' https://localhost:8443/admin/metrics/
```

Also, when the authentication is enabled, Prometheus scrape job
might look like this:

```yaml
  - job_name: 'airflow_exporters'
    metrics_path: /admin/metrics/
    scheme: https
    tls_config:
      insecure_skip_verify: true
    bearer_token: '4e1dba1c-2b66-4275-b8ae-292ee9665fa1'
    scrape_interval: 5m
    static_configs:
    - targets:
      - '127.0.0.1:8443'
      labels:
        environment: 'dev'
    relabel_configs:
    - source_labels: [__address__]
      regex: "^(.*):(.*)$"
      target_label: instance
      replacement: ${1}
    - source_labels: [instance]
      regex: "^(127.0.0.1)$"
      target_label: instance
      replacement: "airflow"
```