prometheus: service labels based on config_ident

[elliefm]

This PR labels metrics with the ident of the service counting them. For example if your cyrus.conf has both "imap" and "imaps" services, the cyrus_imap_foo metric will be split up into cyrus_imap_foo{service="imap"} and cyrus_imap_foo{service="imaps"}, even though the same binary is used for both services.

To avoid reporting metric/service combinations that will never be seen (e.g. cyrus_lmtp_foo{service="imap"}), this PR:

• no longer automatically initialises the last_updated field of metrics during prometheus_init(), instead leaving it as zero; and
• only reports metrics that have a non-zero last updated

A consequence of this is that a newly started Cyrus instance will report very few statistics at all, but the report will gradually thicken out as interesting events occur and are counted.