add prometheus metrics for httpd

[elliefm]

This adds two kinds of metrics to httpd:

• basic connection level stuff (total connections, active connections, ready listeners, shutdown statuses)
• general method/namespace stuff (introspectively)

I expect at some point we will want to enrich the statistics we gather for some protocols that sit on top of httpd -- I'm thinking particularly of JMAP here. There's no reason that each imap/http_[namespace].c file cannot track its own metrics in addition to the generic ones imap/httpd.c tracks: just set them up and count them appropriately. I'd suggest naming such metrics cyrus_jmap_... or whatever, so they're clearly distinct from cyrus_http_...

In the meantime though, here's the bones. I think this is also a pretty good demo of how to add new metrics to services we're not currently tracking metrics from (such as popd).

[ksmurchison]

I don't know anything about prometheus, but these patches look sane.

I could certainly see a desire to keep track of certain REPORT types or POST requests depending on the namespace.

[elliefm]

rebased onto current master and fixed conflicts