Prometheus exporter: incomplete metatags

[vgratian]

Describe the bug

• instance labels (when exported as separate metric) and status metrics do not get the "HELP" and "TYPE" metatags

This was reported by a customer, I will add more details when I get them from the customer and/or reproduce the issues myself.

[vgratian]

I moved the second issue to #132

[cgrinds]

Here's how I validated.

Exporters:
    prometheus2:
        exporter: Prometheus
        port: 12991
        local_http_addr: 0.0.0.0
        add_meta_tags: true
Pollers:
    ocum-mobility-01-02:
        datacenter: nane
        addr: -REDACTED-
        auth_style: password
        username: -REDACTED-
        password: -REDACTED-
        use_insecure_tls: true
        collectors:
            - Zapi
            - ZapiPerf
        exporters:
            - prometheus2

Restarted that poller, waited a minute.
Executed curl -s localhost:12990/metrics | grep "#" | grep Pseudo

# HELP snapmirror_labels Pseudo-metric for snapmirror labels
# HELP aggr_labels Pseudo-metric for aggr labels
# HELP node_labels Pseudo-metric for node labels
# HELP nic_labels Pseudo-metric for nic labels
# HELP shelf_labels Pseudo-metric for shelf labels
# HELP disk_labels Pseudo-metric for disk labels
# HELP volume_labels Pseudo-metric for volume labels