Modernize exporter-toolkit flag parsing

[dswarbrick]

Use modern method of parsing listener address(es) and TLS config file, provided by prometheus/exporter-toolkit.

Refs: #148

Note that this introduces a breaking change.
The following flags:

      --telemetry.address=":9117"  
                           Address on which to expose metrics.
      --web.config=""      Path to config yaml file that can enable TLS or authentication.

will change to:

      --web.listen-address=:9117 ...  
                                 Addresses on which to expose metrics and web interface. Repeatable for multiple addresses.
      --web.config.file=""       [EXPERIMENTAL] Path to configuration file that can enable TLS or authentication.

Additionally, the systemd socket flag will be offered (on Linux):

      --[no-]web.systemd-socket  Use systemd socket activation listeners instead of port listeners (Linux only).

[blkperl]

@Lusitaniae do you have any thoughts on this change?

[Lusitaniae]

in node exporter looks like the toolkit can load the flags directly

prometheus/node_exporter@440a132#diff-7907251477470326df65250bf7fa699e0756ce5d6282e27b34c112132a7ef6b5R162

Also seems we can use it to generate the landing page too
https://github.com/prometheus/node_exporter/pull/2622/files

Maybe need more work here?

[dswarbrick]

@Lusitaniae I'm not sure what you mean by "load the flags directly" or how the node_exporter code differs to what I've proposed - other than the toolkitFlags var being global in this PR, simply because that's where all your other flag vars are.

The landing page functionality in exporter-toolkit is new, and was not available when I opened this PR. Personally, I would recommend keeping that for a separate PR.

[Lusitaniae]

Sounds fair

I'll leave it to @blkperl

[roidelapluie]

This LGTM

[Lusitaniae]

Many thanks

Merged and released in https://github.com/Lusitaniae/apache_exporter/releases/tag/v1.0.0

if you'd like to test