OA-55 Implement a Prometheus Exporter Proxy (similar to the exporter_exporter)

[nook24]

This feature lets the openITCOCKPIT Agent act as a proxy for Prometheus Exporters installed on the same system.
The idea is to have one central HTTP endpoint that can be queried by openITCOCKPIT (the Agent itself) and a /prometheus target that can be scraped by Prometheus.

Exporters like for example the node_exporter do not have to be exposed to the network, instead they can be bind to 127.0.0.1 because the openITCOCKPIT Agent will scrape the exporters.

The new API endpoint /prometheus will return a list of all available exporters

To get the data of a specific exporter, just add a query parameter /prometheus?exporter=mysqld_exporter

If the Agent is using AutoTLS (default) the /prometheus endpoint is encrypted and will also require a TLS certificate for authentication. So everything is secure by default.

The exporter configuration in openITCOCKPIT is pretty straightforward

params:
    exporter: [mysqld_exporter]
scheme: https
tls_config:
    insecure_skip_verify: true
    ca_file: /opt/openitc/agent/server_ca.pem
    cert_file: /opt/openitc/agent/server_ca.pem
    key_file: /opt/openitc/agent/server_ca.key

Currently only Pull Mode is supported, but I think this could also be implemented into the Push Mode.