Add some functionality to the mail collector

[Lacrymology]

This is about the collector I sent in #40

There's one main thing I'd like to add right now that is the possibility of collecting metrics for more than one domain hosted in the server. The main idea behind the mailbox_prefix configuration key was to be able to define more than one instance of the collector, each with a different prefix and path (at least our path to the different domain spools are /var/mail/vhosts/<domain>/), but I just realized that's not possible.

So what do you think would be the best way of configuring this to have:

• from /var/mail/vhosts/example.com collect metrics with prefix example_com
• from /var/mail/vhosts/example.net collect metrics with prefix example.net
• etc

My first thought was to pass /var/mail/vhosts/* as a glob and then automatically add the vhost name as a prefix, but the real distribution inside /var/mail depends on the mail server and configuration, the only thing the standard says is that you should have plain text files named <username> in the UNIX mailbox format for each user's mailbox. But our server has, besides /var/mail/vhosts/<domain>, /var/mail/vhosts/indexes/, and in there there's a lot of random imap information (the different users folders, trash, history, etc)

Another posible approach woud be:

[[MailCollector]]
spool_path = /var/mail         # this will be published without prefix
[[[example_com]]]
spool_path = /var/mail/vhosts/example.com    # prefix=example_com
[[[example_net]]]
spool_path = /var/mail/vhosts/example.net     # prefix=example_net

[kormoc]

Well, with the new functionality to run a collector multiple times, wouldn't it be better to just run it one time per domain with a different path and spool_path set per collector?

[Lacrymology]

wait, I missed this. Sure it would (it was my original idea)

Can you point me to the right spot?

[shortdudey123]

See comments in #675 on running multiple instances of a collector.